Genesis 21:9 - Coke's Commentary on the Holy Bible

Son of Hagar—mocking— St. Paul calls this persecuting, Galatians 4:29 and the original word מצחק metsachek, would lead one to think that Ishmael's ill-usage consisted, in part, of ridicule and abuse of Isaac for his name. The feast was, in fact, the initiation of Isaac, and his father's declaration concerning him, which Ishmael, who thought he had a prior right, was not able to bear; and there is no ground to imagine more, than that this exasperated his rough nature to commit such rudenesses as could not but interrupt the pleasures of the festival, and gave occasion to his own and his mother's expulsion out of the family; she, most probably, inciting and encouraging, or at least justifying her son, in his maltreatments of the heir, the promised and beloved Isaac. The verse does not seem to confine the mocking which Sarah saw, to one particular period; she might possibly see this frequently repeated.