"abstain from food," Old English fæstan "to fast" (as a religious duty), also "to make firm; establish, confirm, pledge," from Proto-Germanic *fastanan "to hold, guard," extended to the religious act "observe abstinence" (source also of Old Frisian festia, Old High German fasten, German fasten, Old Norse fasta "abstain from food"), from the same root as fast (adj.).
The original meaning in prehistoric Germanic was "hold firmly," and the sense evolved via "have firm control of oneself," to "hold oneself to observance" (compare Gothic fastan "to keep, observe," also "to fast"). Perhaps the Germanic sense shifted through use of the native words to translate Medieval Latin observare in its sense "to fast," or it might have been a loan-translation of a Greek expression brought to the Goths by Arian missionaries and spread from them to other Germanic peoples. The verb in the sense "to make fast" continued in Middle English, but was superseded by fasten. Related: Fasted; fasting.
c. 1300, "fixed payment (usually in exchange for taxes collected, etc.), fixed rent," from Old French ferme "a rent, lease" (13c.), from Medieval Latin firma "fixed payment," from Latin firmare "to fix, settle, confirm, strengthen," from firmus "strong; stable," figuratively "constant, trusty" (from suffixed form of PIE root *dher- "to hold firmly, support").
Sense of "tract of leased land" is first recorded early 14c.; that of "cultivated land" (leased or not) is 1520s. A word of confused history, but there is agreement that "the purely agricultural sense is comparatively modern" [Century Dictionary]. There is a set of Old English words that appear to be related in sound and sense; if these, too, are from Latin it would be a very early borrowing. Some books strenuously defend a theory that the Anglo-Saxon words are original (perhaps related to feorh "life").
Phrase buy the farm "die in battle," is from at least World War II, perhaps a cynical reference to the draftee's dream of getting out of the war and going home, in many cases to a peaceful farmstead. The simple term buy it as slang for "suffer a mishap," especially "to die" is attested by 1825, and seems to have been picked up in airmen's jargon. Meanwhile fetch the farm is prisoner slang from at least 1879 for "get sent to the infirmary," with reference to the better diet and lighter duties there.
late 14c., "hallowed, consecrated, or made holy by association with divinity or divine things or by religious ceremony or sanction," past-participle adjective from a now-obsolete verb sacren "to make holy" (c. 1200), from Old French sacrer "consecrate, anoint, dedicate" (12c.) or directly from Latin sacrare "to make sacred, consecrate; hold sacred; immortalize; set apart, dedicate," from sacer (genitive sacri) "sacred, dedicated, holy, accursed." OED writes that, in sacred, "the original ppl. notion (as pronunciation indicates) disappeared from the use of the word, which is now nearly synonymous with L. sacer."
This is from Old Latin saceres, from PIE root *sak- "to sanctify." Buck groups it with Oscan sakrim, Umbrian sacra and calls it "a distinctive Italic group, without any clear outside connections." De Vaan has it from a PIE root *shnk- "to make sacred, sanctify," and finds cognates in Hittite šaklai "custom, rites," zankila "to fine, punish." Related: Sacredness. The Latin nasalized form is sancire "make sacred, confirm, ratify, ordain" (as in saint, sanction). An Old English word for "sacred" was godcund.
The meaning "of or pertaining to religion or divine things" (opposed to secular or profane) is by c. 1600. The transferred sense of "entitled to respect or reverence" is from 1550s. Sacred cow as an object of Hindu veneration is by 1793; its figurative sense of "one who or that which must not be criticized" is in use by 1910 in U.S. journalism, reflecting Western views of Hinduism. Sacred Heart "the heart of Jesus as an object of religious veneration" is by 1823, short for Sacred Heart of Jesus or Mary.