From The Jargon File (version 4.4.7, 29 Dec 2003):
tool
1. n.A program used primarily to create, manipulate, modify, or analyze
other programs, such as a compiler or an editor or a cross-referencing
program. Oppose app, operating system; see also toolchain.
2. [Unix] An application program with a simple, ‘transparent’ (typically
text-stream) interface designed specifically to be used in programmed
combination with other tools (see filter, plumbing).
3. [MIT: general to students there] vi. To work; to study (connotes
tedium). The TMRC Dictionary defined this as “to set one's brain to the
grindstone”. See hack.
4. n. [MIT] A student who studies too much and hacks too little. (MIT's
student humor magazine rejoices in the name Tool and Die.)