argumentum ad hominem

noun logic

fallacious argument that attacks not an opponent's beliefs but his motives or character
argument that shows an opponent's statement to be inconsistent with his other beliefs
an instance of either of these

Word Origin for argumentum ad hominem

literally: argument to the person

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012