c. 1600, Latin, literally "to a man," from ad "to" (see ad-) + hominem, accusative of homo "man" (see homunculus). Hence, "to the interests and passions of the person." Originally an argument or appeal to the known preferences or principles of the person addressed, rather than to abstract truth or logic.
Aristotle (Topics, viii 11) remarks that it is sometimes necessary to refute the disputant rather than his position, and some medieval logicians taught that refutation was of two kinds, solutio recta and solutio ad hominem, the latter being imperfect or fallacious refutation. [Century Dictionary]