To become a son-in-law, to make oneself a daughter's husband, to intermarry
Brown-Driver-Briggs Hebrew Lexicon
1) to become a son-in-law, make oneself a daughter's husband 1a) (Qal) wife's father, wife's mother, father-in-law, mother-in- law (participle) 1b) (Hithpael) to make oneself a daughter's husband