In this case it means "interested." It can also mean "do you agree" in same cases.
We can say things like "are you down for drinks tonight," or "are you down with my plan?"
2010년 5월 12일
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I know there are slang meanings for this word, but when I read the lyrics of this song, it seems to me that the meaning is "unhappy" or "low", as in "I'm feeling down today because my boyfriend dumped me".