In his book That Noble Dream:
The 'Objectivity Question' and the American Historical Profession, Peter Novick takes some time to investigate the term "Ad Hominem."
From what I can remember, nobody can adequately account for the origins of the term, or why it stands for attacking the person and not their arguments.
From the online etymology dictionary we get:
1599, from L., lit. "to a man," from ad "to" + hominem, acc. of homo "man."
From my experience, it is used by the more articulate of right-wing trolls, who, when they advance what they imagine is a profound argument and are surprised to receive demolishing scorn, reach into their grab-bag and accuse their opponent of engaging in "ad hominem attacks."
[My attempt to rise above negativity is floundering.]
I am working on a debate on Canadian policy in Afghanistan. I think I'll try to have it done for Thursday.