Casual sexual activity. Sex while not in a relationship with eachother. Similar to a "one-night stand," but it can happen repetedly. As long as the two people are not serious about eachother, and they have sex, it's hooking up.
Me and John hooked-up last night.
Did you and Mary hook-up at the party?
We aren't going out or anything-- we just hook-up from time to time.
How long have they been hooking up?