Why 'Maundy' Thursday?

The earliest citation of maunde to describe the Thursday before Easter in Middle English dates to the mid-15C.

The word 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, is derived from the Latin mandatum or "commandment".

Biblical reference

From a Christian perspective, the crucial reference was to the opening words of the Latin church service for this day in the church calendar: Mandatum novum do vobis "A new commandment I give unto you" (John xiii:34). 

This new commandment was to love one another.  Its supreme test would be the Passion of Good Friday.

Free read on Medium: The language of Easter? (4 mins)