1. [late 19C+] (also hack off, hell-hack) to irritate, to annoy.
2. [20C+] (US campus) to tease gently.
3. [1970s+] (orig. computing) to tinker with a computer system for pleasure and as a proof of one’s expertise.
4. [1980s+] (orig. computing) to gain unauthorized access to a computer system (and poss. use that access for illegal activities).
5. [2010s] se sense 3 or 4 but with systems other than computers.