But why "Maundy"? Its use is common among traditionalist Catholics as well as Christians of other liturgical churches (e.g. Lutheran, Anglican). I actually haven't heard a complete explanation for the name. There are a few competing theories as to its derivation. The most common explanation seems to be that it derives from the first word of the Latin antiphon from John's Gospel that is sung during the washing of the feet: Mandatum novum do vobis ut diligatis invicem sicut dilexi vos. In English: I give to you a new commandment: that you love one another as I have loved you.
However, the Lutheran Church-Missouri Synod disagrees:
The explanation usually given for the word "Maundy" is popular, even in most dictionaries. But, it is incorrect. According to the popular notion, the term "Maundy" is from the Latin, "mandatum," which means "commandment." Most dictionaries say the same thing, thus proving that there is only one book without errors, and, as you shall shortly see, that one inerrant book isn’t one of most dictionaries.Either way, it's coming!
If the name of this night were to have come from the "mandatum," it would Mandy Thursday, or Mandate Thursday, or even Mandatum Thursday. But, the term comes to us from the Medieval English royal practice of given alms to the poor on this Thursday. There is an English term from the Middle Ages, maund. It is both a noun and a verb. The verb, to maund, comes from the Old French, mendier, which in terms comes from the Latin, mendicare, to beg. To "maund" is to beg. And the noun, maund, refers to a small basket, held out by maunders as they would maund. On Holy Thursday, the extended Royal Family of Great Britain would give alms to the poor prior to attending Holy Thursday mass. These alms were presented to each of the gathered poor in a small velvet bag called the "maundy purse." You see, term Maundy Thursday refers to this practice, and we get this name from the Anglicans, not the Romans, who also now call this day Maundy Thursday as well as Holy Thursday because of the influence of English-speaking Roman churches!