Late Latin, literally "lamb of God." From c. 1400 in English as the name of the part of the Mass beginning with these words, or (later) a musical setting of it. Latin agnus "lamb" is from PIE *agwh-no- "lamb" (see yean). For deus "god," see Zeus. The phrase is used from 1620s in reference to an image of a lamb as emblematic of Christ; usually it is pictured with a nimbus and supporting the banner of the Cross.

1610s, "the killing of a god;" 1650s, "one who kills a god," from stem of Latin deus "god" (see Zeus) + -cida "slayer," from caedere "to kill, to cut down" (from PIE root *kae-id- "to strike").