Appeal to ridicule is a logical fallacy which presents the opponent's argument in a way that appears ridiculous and mocking it.

Examples

1. If Einstein is right that would mean that when I drive my car it gets shorter and heavier. That's crazy!
2. You can't have a cat called Fido. Calling a cat Fido would just be silly!

This should be contrasted with reductio ad absurdum which is a valid logical argument form. Ridiculing the person making the argument is specifically an ad hominem attack.