Why 'Maundy' Thursday?
The first citation of maunde to describe the Thursday before Easter in middle English comes in the mid-15C. It described both The Last Supper and the ceremony of the washing of the feet of the poor or downtrodden.
The immediate origin was the Old French word mandé. This in turn derived from the Latin mandatum or "commandment".
From a Christian perspective, the crucial reference was to the opening words of the Latin church service for this day, Mandatum novum do vobis "A new commandment I give unto you" (John xiii:34).