1. A secret to be kept quiet, from others especially friends or family.
2. The act of homosexual relations between black men. Often unprotected, and/or anonymous sex. Most men do not speak of the sex as they are getting together for a night of poker or other "manly" sport away from their wives or girlfriends.
Jen recently found out she had chlamydia, and she got it from her man Roger. Turns out Rog is on the Downlow with his poker buddies!
(n.) -- the unfortunate view you get when a girl sitting in front of you is wearing very low shorts and a g-string or a thong.
Bad Moon On The Rise is actually a song title, but it fits this phenomenon because the "moon" refers to the girl's rear end, and the whole phrase implies that it's not a generally pleasant thing to look at.
"Ew, look at that girl!"
"Total bad moon on the rise."