The Collaborative International Dictionary of English v.0.48:

Mordant \Mor"dant\, a. [F., p. pr. of mordere to bite; L.
   mordere. See Morsel.]
   1. Biting; caustic; sarcastic; keen; severe.
      [1913 Webster]

   2. (Dyeing & Calico Printing) Serving to fix colors.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mordant \Mor"dant\, n. [F., originally, biting.]
   1. Any corroding substance used in etching.
      [1913 Webster]

   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.
      [1913 Webster]

   3. (Gilding) Any sticky matter by which the gold leaf is made
      to adhere.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mordant
    adj 1: harshly ironic or sinister; "black humor"; "a grim joke";
           "grim laughter"; "fun ranging from slapstick clowning ...
           to savage mordant wit" [syn: black, grim, mordant]
    2: of a substance, especially a strong acid; capable of
       destroying or eating away by chemical action [syn: caustic,
       corrosive, erosive, vitriolic, mordant]
    n 1: a substance used to treat leather or other materials before
         dyeing; aids in dyeing process