From WordNet (r) 3.0 (2006) [wn]:
father-in-law
n 1: the father of your spouse
From The Collaborative International Dictionary of English v.0.48 [gcide]: 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]