What is the guilt by association (bad company) fallacy?

Rejecting a claim merely because some disliked person or group also holds it.

The pattern: a disagreeable person A accepts claim P; therefore P is false. It fails because the company a belief keeps says nothing about its truth — villains and saints alike believe that 2+2=4.

"You think the speed limit should be lowered? Well, that's exactly what that awful politician wants too — so you must be wrong."

Sharing a conclusion with someone unpleasant doesn't make the conclusion false; the claim has to be judged on its own evidence. The fallacy works psychologically because people dislike being associated with those they reject, and may abandon a true belief to avoid the association.

Tip: It's the inverted twin of an appeal to authority — instead of "a good source believes it, so it's true," it's "a bad source believes it, so it's false." Both are irrelevant to the claim.