Ad Hominem

Ad hominem, Latin for "against the man", is a type of logical fallacy in which a person uses a personal attack in lieu of debating an actual idea.

It's more than a simple personal attack, but when a personal attack is used as reasoning for their argument.

An example of an ad hominem attack in action could be the following:

"How can you argue that the world is flat when you don't even brush your teeth or hair?"

Wherein the speaker is using the persons lack of hygiene as evidence that their argument is invalid.