In common usage, "accelerate" means "go faster".

In physics "accelerate" means "change of velocity over time". It doesn't just mean that the velocity increases, just that it changes.

In common usage "velocity" is the same thing as "speed".

In physics "velocity" is "speed in a specific direction". So, "80 km/h" isn't a complete velocity statement. "80 km/h going west" is.

So, a car going at a speed of 80 km/h has a velocity of 80 km/h in the forward direction. Pressing the accelerator causes an acceleration in the forward direction, increasing the speed/velocity. Pressing the brake causes an acceleration in the backward direction, decreasing the speed/velocity. Turning the wheel causes an acceleration in the left/right direction. In this case, the speed might not change, but the velocity changes because the direction of travel changes.

If you imagine blowing on an air hockey puck it's a bit easier to understand. There's a fundamental rule in Physics that F = m × a, force is equal to mass times acceleration. Or, force divided by mass equals acceleration. If you blow on an air hockey puck, you exert a force on it, causing it to accelerate. If the air hockey puck is moving away from you, blowing on it will cause a forward acceleration increasing its velocity. If you blow on it as it's coming towards you, you cause an acceleration backwards, decreasing its velocity. If you blow on it as it's passing by you, you accelerate it sideways. In every case the same F=m × a equation applies, but sometimes the speed gets bigger, sometimes it gets smaller. The trickier one to calculate is when the force causes the direction of travel to change. Then instead of just needing an "x" variable you need "x" and "y", or if you're talking about velocity, vx and vy.

So, in a car, the accelerator increases the engine output which causes a force on the tires that results in a forward acceleration. The brake pedal causes the brakes to exert a force on the tires which results in a backwards acceleration. The steering wheel causes the tires to exert a force on the car accelerating it left or right.

I can imagine a scenario where you a gas is encased in a volume that you can reduce by stepping on it with your foot, thus pressurizing it.

Gasoline is a liquid.