Agnus Dei (n.)
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.