Source : Webster's Revised Unabridged Dictionary (1913)

Son-in-law \Son"-in-law`\, n.; pl. {Sons-in-law}.
   The husband of one's daughter; a man in his relationship to
   his wife's parents.

         To take me as for thy son in lawe.       --Chaucer.

Source : WordNet®

son-in-law
     n : the husband of your daughter
     [also: {sons-in-law} (pl)]