The Collaborative International Dictionary of English v.0.48:

Father-in-law \Fa"ther-in-law`\, n.; pl. Fathers-in-law.
   The father of one's husband or wife; -- correlative to
   son-in-law and daughter-in-law.
   [1913 Webster]

   Note: A man who marries a woman having children already, is
         sometimes, though erroneously, called their
         father-in-law.
         [1913 Webster]

WordNet (r) 3.0 (2006):

father-in-law
    n 1: the father of your spouse

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FATHER-IN-LAW. In latin, socer, is the father of one's wife, or of one's
husband.