A term coined in 1963. Modeled on the much older ad hominem, it refers to attempts to defeat an argument rooted in sexist prejudice against women.  The term is most frequently used in this sense in feminist philosophy, to note systemic tendencies to discredit opinions of women.The practice of course has been around much longer than that.
I was reminded of this because the blogger Digby (a woman but nobody knew that for a long time and many were surprised when she came out of the closet) recently wrote about a right-wing blogger attacking a female political candidate for having large breasts. Apparently he finds them gross. This has nothing to do with her qualities as a politician, but that's typical. Pretty much any woman facing down small-minded creeps will be hit with an ad-feminam attack.
Misogynists will always find some aspect of your appearance to attack. Or they will make references to sexual acts or even make sexual threats.
My own experiences include attacks from anti-abortion protestors, when I was doing clinic defense in the 1990s. I have some of these attacks on videotape - but on VHS so it will be a huge pain to transfer them to digital, but I will have to do it one of these days. Anyway, the very favorite way to attack women doing clinic defense was to tell us we were ugly, and also lesbians. Now beauty is in the eye of the beholder, but I'm about as straight as they come, although totally pro-gay rights. I suspect that they think "lesbian" is just as much of an insult as "ugly."
It's not only hard-core rightwingers though. Ad feminam attacks can come from anywhere. I blogged about it in March of this year - off off Broadway - adventures with creeps.