Hooking up is a process of when a friend hides beside a door and when their friend walks in the room they take a grandfather's cane and pick them up off the ground with the hooked part of the cane in their anus
Sex for its own sake; when one or both of its participants doesn't want or doesn't know if they want a committed relationship but are horny. Usually, sex early on, especially on the first date, is the hallmark sign of a hook-up.
Dude 1: How'd your Tinder date go last night?
Dude 2: Pretty good. We got dinner and drinks and went back to my place.
Dude 1: Nice! Did you hook-up?
Dude 2: Yeah, dude. She was pretty hot.