Some colleagues were recently discussing words and phrases and how they change, and how often the change involves opposites. For instance, you used to be “up for” something if you were eager to do it, or “up on” something about which you were knowledgeable. Now you’re “down with” (or down wit’) something, or you’re just “down.” Something great used to be “sweet,” now it’s “sick” or “dirty.”
If you’re not up on/down with) a phrase you hear, check it out at Urban Dictionary, where you’ll find interesting if sometimes crude reading.