Lit. 'lamb of God'. The phrase used by John the Baptist of Christ, taken from the book of Isaiah 53:7. Later the two words became the opening a part of the mass. The lamb became a familiar icon or symbol of Christ, e.g. in Aethelred the Unready's coinage of c. 1009 and later in both medieval and Byzantine painting.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.