The term “Maundy” comes from the Latin “mandatum;” it is from a verb that means “to give” or “to order” — command. After Jesus and the disciples finished the Last Supper and walked toward Gethsemane, Jesus taught them a new commandment — “I give you a new commandment, that you love one another. Just as I have loved you, you also should love one another” (John 13:34).
The Last Supper
On the first Maundy Thursday, after they had eaten, Jesus gave the disciples his body and blood together with the bread and wine for the forgiveness of their sins. Called Holy Communion or the Eucharist, Lutherans believe this to be one of two sacraments — gifts from God. Most worship services focus on the meal and communion as a way to commemorate this day.
https://www.livinglutheran.org/2013/04/three-days-traditions-maundy-thursday-good-friday-vigil-easter/