Full Idea
The Argumentum ad Hominem is to press a man with consequences drawn from his own principles or concessions.
Gist of Idea
Ad Hominem: press a man with the consequences of his own principle
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.17.21)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.686
A Reaction
This is a rather more plausible account of it than the alternative I have met, that it is just to attack to speaker instead of what they say. This version is at least an attempt to derive a contradiction, rather than mere abuse.