niggardly

Miscellaneous, no one that isn't a native English speaker knows how to pronounce that word

Acknowledge, no one that isn't a native English speaker knows how to write that word

I was just thinking "umbrella" is weird. It just seems so random.

strengths

it breaks so many linguistic rules yet feels just fine to say

Dyslexia - it's hard to spell even if you're not dyslexic.

I think "once" is spelled strangely. In Spanish it's 11 and pronounced as you would expect.

In English the same string of letters is pronounced wonss. Plus the whole once twice thrice for one time two times three times is odd, though at least consistent but then no fourse or anything it just stops.

Onomatopoeia - the word for making a word for a sound

"Fuck"

I think adjective is the only grammar variation it doesn't cover.

Glossolalia

"Kitsch" is hard to define weird. "Absquatulate" is the weirdest word I use on a semi-regular basis because it just means to leave quickly.

Myrrh

Most of the examples here are perfectly cromulent words.

"Winningest"

Vainglorious.

I dunno if it's the weirdest but "pronunciation" is pretty weird.

Why is it "pronUnciation" but "pronOUnce"?

Kumquat

Flabbergasted

onomatopoeia (edit) - the word should have been something akin to soundsalotlikea but no one consulted me.

noun

1. The formation or use of words such as buzz or murmur that imitate the sounds associated with the objects or actions they refer to.
2. The property of a word of sounding like what it represents.
3. A word that sounds like what it represents, such as "gurgle" or "hiss".

Isthmus. I don't claim to know if it's the weirdest, but it's gotta be one of the most difficult to pronounce!

"Weirdest"

Tabernacle

Caveat

"fine"

because it can mean so many different things, like if you say something is fine, it's not very good, but "fine dining" is fancy and good.