Source : Webster's Revised Unabridged Dictionary (1913)

Mordant \Mor"dant\, a. [F., p. pr. of mordere to bite; L.
   mordere. See {Morsel}.]
   1. Biting; caustic; sarcastic; keen; severe.

   2. (Dyeing & Calico Printing) Serving to fix colors.

Mordant \Mor"dant\, n. [F., originally, biting.]
   1. Any corroding substance used in etching.

   2. (Dyeing & Calico Printing) Any substance, as alum or
      copperas, which, having a twofold attraction for organic
      fibers and coloring matter, serves as a bond of union, and
      thus gives fixity to, or bites in, the dyes.

   3. (Gilding) Any sticky matter by which the gold leaf is made
      to adhere.

Mordant \Mor"dant\, v. t. [imp. & p. p. {Mordanted}; p. pr. &
   vb. n. {Mordanting}.]
   To subject to the action of, or imbue with, a mordant; as, to
   mordant goods for dyeing.

Source : WordNet®

mordant
     adj : harshly ironic or sinister; "black humor"; "a grim joke";
           "grim laughter"; "fun ranging from slapstick clowning
           ... to savage mordant wit" [syn: {black}, {grim}]
     n : a substance used to treat leather or other materials before
         dyeing; aids in dyeing process