Maundy Thursday is the traditional English name for the Thursday preceding Easter. The name derives from the Latin version of Jesus’ words in John 13:34:
mandatum novum do vobis ut diligatis invicem sicut dilexi vos ut et vos diligatis invicem,
“A new command I give you: love one another as I love you, you love one another.”
The “new command” is a “new mandate,” thus Maundy Thursday.
Jesus exemplifies this new command in his humble act of washing the disciples’ feet (John 13:5). Jesus issues the “new command” on the same day as the first Lord’s Supper. Thus Maundy Thursday has come to be a commemorative celebration of the institution of the Lord’s Supper.
Source: F. L. Cross and Elizabeth A. Livingstone, eds., The Oxford Dictionary of the Christian Church (Oxford; New York: Oxford University Press, 2005), 1065.