late 14c., past-participle adjective from 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," 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." An Old English word for "sacred" was godcund. 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. Sacred Heart "the heart of Jesus as an object of religious veneration" is from 1765.