[Mythically, from the Latin semi-deponent verb quuxo, quuxare, quuxandum iri; noun form variously ‘quux’ (plural ‘quuces’, anglicized to ‘quuxes’) and ‘quuxu’ (genitive plural is ‘quuxuum’, for four u-letters out of seven in all, using up all the ‘u’ letters in Scrabble).]
1. Originally, a metasyntactic variable like foo and foobar. Invented by Guy Steele for precisely this purpose when he was young and naive and not yet interacting with the real computing community. Many people invent such words; this one seems simply to have been lucky enough to have spread a little. In an eloquent display of poetic justice, it has returned to the originator in the form of a nickname.
2. interj. See foo; however, denotes very little disgust, and is uttered mostly for the sake of the sound of it.
3. Guy Steele in his persona as ‘The Great Quux’, which is somewhat infamous for light verse and for the ‘Crunchly’ cartoons.
4. In some circles, used as a punning opposite of ‘crux’. “Ah, that's the quux of the matter!” implies that the point is not crucial (compare tip of the ice-cube).
5. quuxy: adj. Of or pertaining to a quux.