a girl who is down to participate in sexual activities without prior relationship to said individual
dude let's call over some down heads, get fucked up and smash
Depending on the rest of the sentence i.e., context; depressing, depression, or depressing event or situation. Bummed out, bummer.
1) Aw dude ... you're giving me a down head! (You are depressing me)
2) Sorry to hear that ... that's a down head. (That is a depressing event, or bummer)
3) I'm in a down head (bummed-out mood ... though it is rarely used in this way, I would certainly understand what the person meant if s/he said this)