What does Genesis 17:18 mean?

Abraham found God's promise in the previous verse inconceivable. In fact, he literally found it fall-down-laughing funny. It was beyond imagining that a 100-year-old man and a 90-year-old woman could have a baby. In Abraham's mind, it was also unnecessary. He had a son, though not through Sarah. Ishmael was a young man of 13. His birth mother Hagar had received a promise from God that Ishmael's descendants would be exceedingly numerous, just as God had promised Abraham.

Abraham could picture it all. He could see Ishmael, his son, becoming the nations God had promised, receiving God's blessing, fulfilling God's plan. "If only," Abraham said to God. "If only Ishmael could live under your blessing!" In the next verse God will respond to Abraham. In a gracious example of gentle correction, God will not reply with "no," but with a "yes, but…"

God's response is gentle, but it is not without some level of irony. In it, God will declare a name for this soon-to-be-son, one which will forever remind Abraham of his reaction. The boy is to be named Isaac, which literally means "he laughs."