Full Idea
The Ad Hominem Fallacy is to criticise the person proposing an argument rather than the argument itself, as when you say "You would say that", or "Your behaviour contradicts what you just said".
Clarification
'Ad Hominem' is Latin for 'against the man'
Gist of Idea
The Ad Hominem Fallacy criticises the speaker rather than the argument
Source
PG (Db (ideas) [2031])
Book Reference
A Reaction
Nietzsche is very keen on ad hominem arguments, and cheerfully insults great philosophers, but then he doesn't believe there is such a thing as 'pure argument', and he is a relativist.