When a person you, (or unknown) wants to call/text you. But mostly in a friendly way.
"Hit me up when we get home from school."
For a woman to to add a man's contact (vise versa) in order to fuck him, and then ditch man and tell everyone that he raped the woman (False rape accusations).
October 30th is national hit up your ex day.
“Hey mark, wanna get back together for national hit up your ex day?”