Down With 'Down With'

In an old Simpsons episode, a group of feminist protesters marches in front of an ad agency, presumably protesting some chauvinistic ad campaign. The women are carrying signs and chanting, "Down with sexism! Down with sexism!" In what turns out to be a spoof beer commercial, some construction workers spray them with beer and they turn into giggling bikini party girls. (So like life, those Simpsons.)

This well-known usage of "down with" appears in Webster's New World College Dictionary, curiously flush with exclamation points:

down with! overthrow! do away with!

Yet the online version of the American Heritage Dictionary also lists that other, hipper version of "down with":

Slang. Having knowledge of; aware; "He was not, I detected, 'down with the revolution'" (Clarence Page).

And urbandictionary.com reports the next phase of this term's evolution:

down wit dat: to be in agreement

And here we have yet another example of why native English speakers should go easy on anyone trying to learn this language: The protesters who were originally "down with!" sexism changed their minds and instead became "down with" sexism.