Ad hominem

Ad hominem means "against the man", rather than against what the man is saying. This Latin phrase describes one of the oldest known logical fallacies.

It casts doubt on what a person is saying because of (imputed) flaws in the person which are unrelated to the matter at hand.

For example, the statement 'Richard Wagner was an opera composer much admired by the Nazis' might be considered an ad hominem attack on Wagner's music. It is true, and in some contexts might be a relevant comment. But if someone says it in order to suggest that Wagner was a bad composer, it's an ad hominem attack, because whether some bad people liked his music is not a relevant comment on the quality of his music.

As another example, the statement 'The atheist Karl Marx believed in taking wealth from the rich and redistributing it to the poor' might be considered an ad hominem attack on the policy of income redistribution. Karl Marx may or may not have been right to advocate income redistribution; but whether he was an atheist is irrelevant to that question. So if the speaker mentions that he is atheist (to someone who considers that a bad thing) when the topic under discussion is income distribution, not religion, then that's an ad hominem attack.