The Collaborative International Dictionary of English v.0.48:

Maranatha \Mar`a*nath"a\, n. [Aramaic m[=a]ran ath[=a].]
   "Our Lord cometh;" -- an expression used by St. Paul at the
   conclusion of his first Epistle to the Corinthians (xvi. 22).
   This word has been used in anathematizing persons for great
   crimes; as much as to say, "May the Lord come quickly to take
   vengeance of thy crimes." See Anathema maranatha, under
   Anathema.
   [1913 Webster]