1. Something that has unpleasant smell.
2. Something that looks nasty/raunchy.
3. Something that you don't want to do, that is lame/time wasteful.
Definition 1. "Ew maine! Yo armpits are RANNK!"
Definition 2. "Dude, yo bathtub needs some scrubbin'! Talk about rank!"
Definition 3. "Damn dawwwg, this homework is rank!"