I think others have answered your question better than me, but I'll chip in my two cents anyway.

The definition depends on who is saying it.

Within mainstream US politics, Republicans use "liberal" as a catch-all pejorative for any person or group further to the left of themselves. It is usually aimed at Democrats but could also refer to Greens, communists, etc.

The irony is that, in a broader political context, Republicans are very much liberals, too. People outside the US political mainstream who sneer about "liberals" are usually referring to this larger group, which basically encompasses the capitalist status quo in the "western" world.

It means "anyone who doesn't immediately agree with anything I say/repeat" in magaspeak.

Using liberal as a derogatory says far more about the person saying it, than it does about those they say it about.

My advice is to not take them seriously.

It's an insult in the US because the liberals in the Democratic party are considered to be on the right side of the political spectrum. Today's liberals are seen as pushing the same stuff Republicans were pushing in the era of Reagan and Bush. Progressives in the party don't really get much of a voice therefore they disparage the liberals that keep pushing the Overton window further and further right.

Sigh, I'll wade into this river of shit.

Liberalism is broadly understood as neoliberalism, which is an ideological descendant from classical liberalism. This ideology positions itself as being broadly in favour of individual freedom within a rather tight definition of freedom. Namely liberals are concerned with the ability of people to read what they like, own what they like, marry whomever they like and so on provided they do this inside of a system of capitalist free market exchange.

Modern liberalism tends to frown on heavy government intervention in market affairs, which they see as representing the free (and thus good) exchange of goods between individuals. They also tend to be broadly in favour of the militaristic western global hegemony.

Criticism of this attitude comes from 2 places.

1. too much freedom.

2. not enough freedom.

(1) is people that want women bound up in the kitchen and walk around with an odd gait that makes you remember Indiana Jones films

(2) are people (I'm in this camp) who see liberalism as a weak ideological position that favours stability over justice and, in so doing, ignores the suffering of billions.

Basically it says to me “Fuck your for being/showing/reminding me that you are smarter/more informed/more reasoned/more compassionate than me!”

Same applies to “woke”.

It’s how it’s said that speaks volumes.

Phil Ochs says it better than I can:

In every American community there are varying shades of political opinion. One of the shadiest of these is the liberals. An outspoken group on many subjects, ten degrees to the left of center in good times, ten degrees to the right of center if it affects them personally.

The issues have changed slightly from the 1960s, but his song's accusations of hypocrisy and NIMBYism among those who publicly espouse progressive causes still hit close enough to home.

I'm not really into this stuff, but from what I can tell, liberal became an insult purely because conservatives wanted to label liberals as inferior. That's about it. Some leftists also use liberal as an insult, though to a lesser extreme.

How conservatives use it, liberal means someone that is too sensitive, accommodating, weak, and evil (especially in Christian terms).

How leftists use it, liberal means uneducated, hypocritical, insufficient, and turncoat/traitor.

The epitome of Shit Americans Say.

I'll go more general, any label can be a insult. In spoken language you can hear it in the tone, the pitch, and the slurring of the word. Spoken with hate.

The normal rhetorical method is to use a extreme caricature of the opponents ideas and misapply the label, use it in derisive context... and overtime it becomes a commonly understood insult.

example: Those damn pescatarians! They are so focused on the sea they cannot see where they stumble on the land! Bloody navieish pescatarians.

There are a lot of solid answers, but it's good to remember that the US definition of a liberal, seems to be very different from the international definition. So, remember to keep context in mind, and whether it's coming from an American or not.

Why liberalism and leftism are increasingly at odds, written by a liberal. (Note that by "SJL" he means modern American "social justice leftism" as opposed to earlier leftist movements more focused on economic class.)

SJLs and liberals have some interests in common. Both are “culturally liberal” on questions like abortion and gay marriage. And both disdain Donald Trump and the modern, MAGA-fied version of the Republican Party. But I’d suggest we’ve reached a point where they disagree in at least as many ways as they agree. Here are a few dimensions of conflict:

SJL’s focus on group identity contrasts sharply with liberalism’s individualism.

SJL, like other critical theories that emerged from the Marxist tradition, tends to be totalizing. The whole idea of systemic racism, for instance, is that the entire system is rigged to oppress nonwhite people. Liberalism is less totalizing. This is in part because it is the entrenched status quo and so often is well-served by incremental changes. But it’s also because liberalism’s focus on democracy makes it intrinsically pluralistic.

SJL, with its academic roots, often makes appeals to authority and expertise as opposed to entrusting individuals to make their own decisions and take their own risks. This is a complicated axis of conflict because there are certainly technocratic strains of liberalism, whereas like Hayek I tend to see experts and central planners as error-prone and instead prefer more decentralized mechanisms (e.g. markets, votes, revealed preferences) for making decisions.

Finally, SJL has a radically more constrained view on free speech than liberalism, for which free speech is a sacred principle. The SJL intolerance for speech that could be harmful, hateful or which could spread “misinformation” has gained traction, however. It is the predominant view among college students and it is becoming more popular in certain corners of the media and even among many mainstream Democrats.

As a self-idenitied liberal, I think this is about right. He also talks about how and why the conflict in the Middle East has caused the recent increase in hostility but I'm not going to quote the whole article.

Leftists have always despised liberalism - and for pretty damn obvious reasons for anyone that has a basic understanding of what left and right actually means. Liberalism is a right-wing ideology that upholds capitalism, colonialism and white supremacism, all while having a recurring habit of enabling and supporting fascism at the drop of a hat when the precious liberal order is threatened by change from below.

So yeah - as far as leftists are concerned, the term liberal is as much of an insult as the term fascist is. There is even an old leftist saying, "kill the liberal inside your head" which sums up pretty nicely what leftists think of liberals and liberalism.

Liberalism is a right wing ideology, which because America is so right shifted is seen as left in their country or at least a counterpoint to conservatism when they’re basically the same thing - deregulate, don’t tax, “free market” capitalist nonsense that’s destroying the planet.

So from a left wing perspective it’s a derogatory term. Most leftists take on a more worldly understanding, so even American leftists will use it this way.

As for American right wingers, they’re just using it for hate. I doubt they could even explain who they mean.

I was going to try to explain it then I made the mistake of reading the complete insanity in the comments. Think I’ll just move on. Hopefully you get your answer.

In the USA. As others in the thread have said…

It is meant to ”other” — “You” are a good person, loves your Mama; loves Jesus and America too… ( miss you Tom Petty)

But “they” are other.. those dang coastal elites they are latte sipping LIBERALS and hate everything about you.

It identifies them as wanting to take everything away from you. They don’t love Jesus , not the way you do. They want EVs forced on everyone. Etc