c.1300, from past participle of obsolete verb sacren "to make holy" (early 13c.), from Old French sacrer (12c.), from Latin sacrare "to make sacred, consecrate," from sacer (genitive sacri) "sacred, dedicated, holy, accursed," from Old Latin saceres, which Tucker connects to base *saq- "bind, restrict, enclose, protect," explaining that "words for both 'oath' & 'curse' are regularly words of 'binding.' " But Buck merely groups it with Oscan sakrim, Umbrian sacra and calls it "a distinctive Italic group, without any clear outside connections."
Nasalized form is sancire "make sacred, confirm, ratify, ordain." Sacred cow "object of Hindu veneration," is from 1891; figurative sense of "one who must not be criticized" is first recorded 1910, reflecting Western views of Hinduism.