Literally, to apply a powdered cosmetic to one's nose in order to reduce the amount of shimmer or shine.
More commonly a euphemism used by women for going to the ladies' room.
When used by a man it is a very obvious hint that he is tired of the company he is with.

Even more commonly (in the right company), used as a euphemism for "I'm going to go snort a line of [insert white drug here]." This line was used in the flick Pulp Fiction, but that was not its first use in this context.