Whilst in an arguement, to avoid the point (thus a cop-out) and to insult the person arguing. Often because the person who results to the insults has no logical/intelligent point, thus they break down to immaturity.
Jon: I've made my point five times and all you've done is call me 'gay', way to resort to ad hominem, dude.
An attack upon an opponent in order to discredit their arguement or opinion. Ad hominems are used by immature and/or unintelligent people because they are unable to counter their opponent using logic and intelligence.
Person A: I think we should spend more money on environmental protection.
Person B: You just think that because you’re a stupid tree-hugger.
Person A: It is crucial that we facilitate adequate means to prevent degradation that would jeopardize the project.
Person B: You think that just because you use big words makes you sound smart? Shut up you loser; you don't know what you're talking about.
Originally from the Latin meaning - "argument to the man", "argument against the man".
Now meaning Argument against what a person has posted via the net and brought about by Psychological projection as to the person hidden behind the Computer Screen and On-line Identity.
Can also be seen as an idiotic lack of appreciation of Netiquette and also gross self absorption by the reader/responder.
"Dear @$£*&% I read your post and am shocked by the Ad Hominem way you respond to reasonable dialog in this forum.... etc etc."