1. n. Originally, a quick job that produces what is needed, but not well.
2. n. An incredibly good, and perhaps very time-consuming, piece of work that produces exactly what is needed.
3. vt. To bear emotionally or physically.
"I can't hack this heat!"
4. vt. To work on something (typically a program). In an immediate sense:
"What are you doing?"
"I'm hacking TECO."
In a general (time-extended) sense:
"What do you do around here?"
"I hack TECO."
More generally, "I hack 'foo'" is roughly equivalent to "'foo' is my major interest (or project)".
"I hack solid-state physics."
5. vt. To pull a prank on.
See sense 2 and hacker (sense 5).
6. vi. To interact with a computer in a playful and exploratory rather than goal-directed way.
"Whatcha up to?"
"Oh, just hacking."
7. n. Short for hacker.
8. See nethack.
Constructions on this term abound. They include 'happy hacking' (a farewell), 'how's hacking?' (a friendly greeting among hackers) and 'hack, hack' (a fairly content-free but friendly comment, often used as a temporary farewell).