Charles Ellicott Commentary Genesis 17:19

"And God said, Nay, but Sarah thy wife shall bear thee a son; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant for his seed after him." — Genesis 17:19 (ASV)

Indeed. —In the Hebrew this word comes first, and is intended to remove all doubt or desire for any other turn of affairs. It should be rendered, And God said, For a certainty Sarah your wife shall bear you a son.

You shall call his name Isaac. —That is, he laughs. The name was to be a perpetual memorial that Isaac’s birth was naturally such an impossibility as to excite ridicule.