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.