(1) The LORD took note of Sarah as God had promised, and the LORD did for Sarah as God had spoken. (2) Sarah conceived and bore a son to Abraham in his old age, at the set time of which God had spoken. (3) Abraham gave his newborn son, whom Sarah had borne him, the name of Isaac.
“Who would have said to Abraham
That Sarah would suckle children!
Yet I have borne a son in his old age.” (8) The child grew up and was weaned, and Abraham held a great feast on the day that Isaac was weaned.
(1) מצחק MAKING SPORT — This means worshipping idols, as it is said in reference of the Golden Calf, (Exodus 32:6) “And they rose up to make merry (לצחק).”
Another explanation is that it refers to immoral conduct, just as you say in reference to Potiphar’s wife, (Genesis 39:17) “To mock (לצחק) at me.”
Another explanation is that it refers to murder, as (2 Samuel 2:14) “Let the young men, I pray thee, arise and make sport (וישחקו) before us” (where they fought with and killed one another)
From Sarah’s reply — “for the son of this bondwoman shall not be heir with my son״ — you may infer that he (Ishmael) was quarrelling with Isaac about the inheritance, saying, “I am the first-born and will, therefore, take a double portion”. They went into the field and he (Ishmael) took his bow and shot arrows at him (Isaac), just as you say (Proverbs 26:18-19) “As a madman who casts firebrands, [arrows and death] and says: I am only מצחק mocking” (Genesis Rabbah 53:11).
(2) מצחק, making fun of the big party Avraham had given to mark the weaning of Yitzchok, Ishmael claimed that surely Sarah must have become pregnant from Avimelech. The reason he had not made such remarks already at the time when Yitzchok had been born, was because he had only overheard wicked gossip about this at a later stage, and now he repeated what he had heard.
(1) ויאמר...על הנער ועל אמתך, God knew that Avraham was not only displeased about Sarah’s request to expel Ishmael, but he was also chagrined at being asked to expel Hagar, his maid-servant. [described here as his servant rather than as Sarah’s servant. Ed.] Even though the Torah had only spelled out that Avraham had felt aggrieved on account of his son (verse 11), the reason why our text had mentioned only his son was that he was his principal concern. He was prepared to quarrel with Sarah about the future of his son, whereas he was not prepared to make the continued presence of Hagar an issue between them. God, however, knew how he really felt concerning Hagar.