An adjective, verb, noun, whatever the hell you want it to be.
A) used when referring to someone's dick
B) when someone gets hit (anywhere, not just in the sack)
C) when something that sucks some serious ass happens to somebody
A) What a small cack Derek has!
B) Oh snap, that brick just cacked him in the face.
C) Your girlfreind cut off your dick? That's a cack and a half dude.