What is the X in X-mas?
Old English catt (c.700), from West Germanic (c.400-450), from Proto-Germanic *kattuz (cf. Old Frisian katte, Old Norse köttr, Dutch kat, Old High German kazza, German Katze), from Late Latin cattus.
The near-universal European word now, it appeared in Europe as Latin catta (Martial, c.75 C.E.), Byzantine Greek katta (c.350) and was in general use on the continent by c.700, replacing Latin feles. Probably ultimately Afro-Asiatic (cf. Nubian kadis, Berber kadiska, both meaning "cat"). Arabic qitt "tomcat" may be from the same source. Cats were domestic in Egypt from c.2000 B.C.E., but not a familiar household animal to classical Greeks and Romans. The nine lives have been proverbial since at least 1560s.
The Late Latin word also is the source of Old Irish and Gaelic cat, Welsh kath, Breton kaz, Italian gatto, Spanish gato, French chat (12c.). Independent, but ultimately from the same source are words in the Slavic group: Old Church Slavonic kotuka, kotel'a, Bulgarian kotka, Russian koška, Polish kot, along with Lithuanian kate and non-Indo-European Finnish katti, which is from Lithuanian.
Extended to lions, tigers, etc. c.1600. As a term of contempt for a woman, from early 13c. Slang sense of "prostitute" is from at least c.1400. Slang sense of "fellow, guy," is from 1920, originally in U.S. Black English; narrower sense of "jazz enthusiast" is recorded from 1931.
Cat's paw (1769, but cat's foot in the same sense, 1590s) refers to old folk tale in which the monkey tricks the cat into pawing chestnuts from a fire; the monkey gets the nuts, the cat gets a burnt paw. Cat bath "hurried or partial cleaning" is from 1953. Cat burglar is from 1907, so called for stealth. Cat-witted "small-minded, obstinate, and spiteful" (1670s) deserved to survive. For Cat's meow, cat's pajamas, see bee's knees.
1975, medical acronym for computerized axial tomography or something like it. Related: CAT scan.
computerized axial tomography
A bulldozer or Caterpillar tractor
[1940s+; fr Caterpillar, trademark for a kind of continuous-track tractor]
A catamaran boat (1960s+)
A Cadillac: Tia Juana pulled up in his long green Cat (1940s+ Black)
Common Abstract Tree Language. R. Voeller & Uwe Schmidt, U Kiel, Germany 1983. Universal intermediate language, used by Norsk Data in their family of compilers. "A Multi-Language Compiler System with Automatically Generated Codegenerators, U. Schmidt et al, SIGPLAN Notices 19(6):202-2121 (June 1984).
(From "catenate") Unix's command which copies one or more entire files to the screen or some other output sink without pause.
See also dd, BLT.
Among Unix fans, cat is considered an excellent example of user-interface design, because it delivers the file contents without such verbosity as spacing or headers between the files (the pr command can be used to do this), and because it does not require the files to consist of lines of text, but works with any sort of data.
Among Unix haters, cat is considered the canonical example of *bad* user-interface design, because of its woefully unobvious name. It is far more often used to blast a file to standard output than to concatenate files. The name "cat" for the former operation is just as unintuitive as, say, LISP's cdr.
Of such oppositions are holy wars made.