‘Maundy’ is derived from the latin ‘mandatum’ which means basically “commandment.”
Because Thursday night of Holy Week corresponds to the Last Supper, it includes Jesus saying, “A new commandment I give to you, that you love one another as I have loved you.” This is the night of that New Commandment, in other words, it is "New Commandment Thursday".
'What is Maundy Thursday?'
Almighty Father, whose most dear Son, on the night before he suffered, instituted the Sacrament of his Body and Blood: Mercifully grant that we may receive it in thankful remembrance of Jesus Christ our Savior, who in these holy mysteries gives us a pledge of eternal life; and who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.