Hooking up means having some sort of intercourse, whether it's oral anal or vaginal. Having oral, anal, or vaginal intercourse, usually spur of the moment with a new partner. Hooking up means any sexual act with a partner of either gender.
What happens during a hookup?
In general, hooking up means being intimate/sexual with someone without being in a committed relationship with them. It may also be called “a one-night stand” or “casual sex.” Hookups may be a one-time thing or something that happens more than once with the same person/people.