[very common]
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.” See Hacking X for Y.
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.
9. [MIT] v. To explore the basements, roof ledges, and steam tunnels of a large, institutional building, to the dismay of Physical Plant workers and (since this is usually performed at educational institutions) the Campus Police. This activity has been found to be eerily similar to playing adventure games such as Dungeons and Dragons and Zork. See also vadding.
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). For more on this totipotent term see The Meaning of Hack. See also neat hack, real hack.