Well, here's one explanation.
Or several.
"Maundy Thursday" is the usual name for this day in England, which comes from the Latin word mandatum meaning "commandment"<1> or the Latin word mendicare meaning "beg".<2> "Maundy Thursday" is therefore the usual name also in Protestant churches that originated in that country and even in some that originated in Scotland, although the Scottish Book of Common Prayer uses the name "Holy Thursday."<3> On the other hand, the United Methodist Book of Worship uses only "Holy Thursday" as a name for the day,<4> though both names are used informally.<5> Other English-speaking Protestants, such as Lutherans, use both "Maundy Thursday" and "Holy Thursday".<6> In the official English liturgical texts of the Roman Catholic Church, even in England, the name for the day is "Holy Thursday".<7><8><9> This is also the name used by the Vatican, as in the series of letters that Pope John Paul II wrote to priests each year on "Holy Thursday" and in the list of liturgical celebrations by the Pope. In the Eastern Orthodox Church, the name for the holy day is Great and Holy Thursday.<10>
According to a common theory, the English word Maundy in that name for the day is derived through Middle English, and Old French mandé, from the Latin mandatum, the first word of the phrase "Mandatum novum do vobis ut diligatis invicem sicut dilexi vos" ("A new commandment I give unto you, That ye love one another; as I have loved you"), the statement by Jesus in the Gospel of John (13:34) by which Jesus explained to the Apostles the significance of his action of washing their feet. The phrase is used as the antiphon sung during the "Mandatum" ceremony of the washing of the feet, which may be held during Mass or at another time as a separate event, during which a priest or bishop (representing Christ) ceremonially washes the feet of others, typically 12 persons chosen as a cross-section of the community.
According to other authorities, the English name Maundy Thursday arose from "maundsor" baskets, in which on that day the king of England distributed alms to certain poor at Whitehall: "maund" is connected with the Latin mendicare, and French mendier, to beg.<2><11> A Lutheran Church-Missouri Synod source likewise states that, if the name were derived from the Latin mandatum, we would call the day Mandy Thursday, or Mandate Thursday, or even Mandatum Thursday; and that the term "Maundy" comes in fact from the Latin mendicare, Old French mendier, and English maund, which as a verb means to beg and as a noun refers to a small basket held out by maunders as they maunded. The name Maundy Thursday thus arose from a medieval custom whereby the English royalty handed out "maundy purses" of alms to the poor before attending Mass on this day.<12>
http://en.wikipedia.org/wiki/Maundy_Thursday