UPDATE: The Bibliography attached to the Wikipaedia entry suggests that the "modern" usage is not found earlier than 1986; and that the Lockean meaning still held force in Fowler, 1926. Whoever wrote that entry appears not to have heard of Locke or to have read much literature from before the 1990s. This exemplifies another cultural problem: the erecting of barriers between the ages. C S Lewis attributes this to the activity of devils. It probably also exemplifies a decline in the study of the Classics. Another C S Lewis point ...
I seem to keep seeing, day after day, the phrase Argumentum ad hominem misused. A recent example occurred in Fr Tom Weinandy's otherwise splendid Letter to PF.
People use it now, apparently, to mean "a personal attack". That is, when you attack somebody in a violent and deeply personal way, rather than arguing politely and rationally about a question in hand. That is how Fr Tom uses it; his point is that PF and the Bergoglians refuse polite dialogue and simply hurl nasty hate-filled personal abuse around. And, of course, he's dead right. That is exactly what they do. But this is not what Argumentum ad hominem means.
Well, language changes. If enough people use Argumentum ad hominem in this incorrect sense, then I suppose one will, regretfully, have to stop calling it wrong. Usage validates. Every philologist knows that.
But I think it is a great shame that an elegant and well-observed description of a certain sort of precise argument is being taken over and forced to mean something crude which is totally different. Something useful is being lost in the field of human discourse, with no apparent compensating advantage that I am capable of discerning.
And ... I am sorry to be personal!! ... I greatly mistrust the motives of some who misuse the phrase. I think the poor things sometimes do it because they think it sounds fine and dandy to say something in Latin. I think saying something in Latin, when you think it means something quite the opposite to what it really means, is embarrassingly pretentious and, to be frank, a display of ignorance. Why not just say "You are making this attack rather personal"? What harm is there, for heaven's sake, in speaking English? It's a very respectable language ... the language of Jane Austen and Ronald Knox and C S Lewis and etc.etc..
So what really is an Argumentum ad hominem? A proper one, in its true native habitat?
Here is Locke's very neat definition: "To press a man with consequences drawn from his own Principles and Concessions".
I've written about this before, giving examples from Socrates to Newman. You could find my earlier blogposts via the search Engine attached to this blog, sub voce Argumentum ad hominem ... if you were interested.