The Collaborative International Dictionary of English v.0.48:

Maundy Thursday \Maun"dy Thurs"day\ [OE. maunde a command, OF.
   mand['e], L. mandatum, from mandare to command. See called
   from the ancient custom of washing the feet of the poor on
   this day, which was taken to be the fulfillment of the "new
   commandment," --John xiii. 5, 34.] (Eccl.)
   The Thursday in Passion week, or next before Good Friday.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Maundy Thursday
    n 1: the Thursday before Easter; commemorates the Last Supper
         [syn: Maundy Thursday, Holy Thursday]