Agnus Dei (n.) Look up Agnus Dei at Dictionary.com
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.