Ramban, on Gen 17:17

“Abraham fell upon his face, and laughed, saying in his heart: Shall a son, thinkest thou, be born to him that is a hundred years old? and shall Sara that is ninety years old bring forth?”

“‘VAYITZCHAK’ (AND HE LAUGHED). Onkelos translated: “and he rejoiced.” This is correct since the word tzachak can be used intermittently for sporting or rejoicing, just as in the verses: ‘Mesacheketh’ (Sporting) in His habitable earth, ‘mesachkim’ (rejoiced) before the Eternal. In my opinion the intent of this expression is to convey the thought that whoever sees a favorable unusual event in one’s life rejoices to the point where “his mouth is filled with laughter.” It is this thought which Sarah expressed: G-d hath made laughter for me; everyone that heareth will laugh on account of me, just as in the verse, Then was our mouth filled with laughter, and our tongue with singing. And this is what Abraham did. When this good tiding was related to him, he rejoiced and his mouth was filled with laughter, and he said in his heart that this is an occasion for rejoicing as it is a very wonderful matter. Shall a child be born unto him that is a hundred years old? and shall Sarah, that is ninety years old, bear? And will this matter not give cause for rejoicing and happiness? Only, Scripture speaks summarily of his wonderment, as it is connected with the word vayitzchak. A similar case is the expression, Have I even here seen, which is connected with the beginning of the verse: for she [Hagar] said, “Have I even here seen G-d, revealing to me that He saw in my affliction, and shall I not call Him E-il-ro’i [the G-d who seeth me]?” Another such example is the expression, Did I reveal Myself unto the house of thy father, which is connected with the following verse, Why do you kick at My sacrifice and at Mine offering,, Verse 29. thus stating, “Have I chosen you so that you kick at My sacrifice and Mine offering? Why then do you do so?” It may be that the question, Shall a child be born to him that is a hundred years old? is an expression of amazement, not one of impossibility, just as in the verse, Wilt thou judge the bloody city, which means, “Do you want to judge her and cause her to know all her abominations?” A similar case is the verse, Wilt thou set thine eyes upon it? it is gone. Likewise is the verse, Hast thou eaten of the tree, whereof I commanded thee that thou shouldst not eat? That is to say, “Has the thought occurred to you to eat of the tree?” This verse likewise is saying, “Has the thought occurred to anyone that a child be born to him that is a hundred years old, and that Sarah, who is ninety years, shall bear?” After that Abraham said to Him that he wishes this miracle be with the life of Ishmael. SHALL A CHILD BE BORN TO HIM THAT IS A HUNDRED YEARS OLD? There is no wonder that a person who is a hundred years old should beget children, for men beget as long as they have vigor even if they be ninety years old or a hundred years old, even in these generations. All the more in the days of Abraham, who had not yet lived two thirds of his life. Moreover, forty years after this, he begot many children from Keturah. But Abraham’s saying, Shall a child be born to him that is a hundred years old, expressed this thought: Since he did not beget a child from this woman while he was young, how could he beget from her after he was a hundred and she ninety? He knew that her womanly periods had ceased. Therefore Abraham did not say so originally when he was told, And I will make nations of thee, but only when he was told that he would beget a child from Sarah. Now He commanded him, And thou shalt call his name Isaac, on account of the rejoicing done by Abraham, which is proof that it was out of faith and joy [that Abraham said, Shall a child be born, etc.] And after Abraham called him so, as G-d had commanded him, Sarah said, “Is he not rightly named Isaac? For G-d hath made laughter for me.””

Imported from an open dataset — not yet checked against the printed edition.