Meaning

noun a song or hymn of mourning composed or performed as a memorial to a dead person

noun a musical setting for a Mass celebrating the dead

noun a Mass celebrated for the dead

originFrom Middle English requiem, from Latin requiem, the first word of the introit for the traditional requiem mass, an alternative accusative case of Latin requiēs (“rest, repose”), from re- (“again”) + quiēs (“rest, quiet”).

Etymology from Wiktionary, CC BY-SA 3.0