🔼The name Isaac in the Bible
Isaac gets quite a bit of Biblical screen time in the New Testament (his name is spelled Ισαακ in Greek). A cool two thousand years after Isaac breathed his last, Jesus proclaimed him, his father and his son as being alive and well, and not dead (Luke 20:37). Isaac is mentioned twice in relation to Abraham's willingness to sacrifice him on Mount Moriah (Hebrews 11:17, James 2:21), and he is celebrated as the child of the promise three times (Romans 9:7, Galatians 4:28, Hebrews 11:18).
🔼Etymology of the name Isaac
The name Isaac is usually spelled יצחק and sometimes ישחק. It comes from the verbs צחק (sahaq) and שחק (sahaq) meaning to laugh:
Abarim Publications' Theological DictionaryLoading: צחק (or click this link)
The question whether Sarah laughed out of joy or out of mockery (Genesis 18:12) is usually answered in her disfavor. Wholly unjust, is our opinion, here at Abarim Publications.
Sure, Sarah laughed within herself and she denied she did, perhaps because she was trying to be polite, or perhaps because she was afraid that she might get yelled at. But Abraham laughed as well and for the same reason, namely out of gladness for becoming a father (Genesis 17:17; Jones' Dictionary of Old Testament Proper Names refers to this verse to explain the name Isaac).
Being childless is a great grief, especially in those days and especially for women. But Abraham and Sarah had received a great promise some time before: they were going to be parents for sure, even the arch-parents of a great nation.
To mark that promise, Abraham had circumcised himself, his son Ishmael and the entire village that he ruled. And to put that in perspective: previously, Abraham waged a war with 318 men that were born in his house (Genesis 14:14). Add to them their elderly fathers, younger brothers and an untold number of foreign-born men and you have an idea of what an operation this would have been. Abraham must have been an incredibly persuasive man to get this all done. No man will surrender a piece of his penis without having been presented with an exceptionally good reason, and no woman will have her husband incapacitated for quite some time (Genesis 34:25) for just an airily inkling.
Sarah, being Mrs. Mayor, was most likely busy for months appeasing the women folk, explaining what was going on and the whole enterprise was certainly impossible to not pay heed to or even forget.
On top of that, Sarai was re-named Sarah, so that every time someone called her, she was reminded of the fact that God promised her a child. And when she overhears a visiting stranger assure her husband that she will be a mother same time next year, her heart leaps for joy and she laughs, just like Abraham.
The stranger's response is full of encouragement and kindness: "Go ahead. Be glad. Be openly happy". When Isaac is born, Sarah says, "God has made laughter for me; everyone who hears will laugh with me". (Genesis 21:6).
Centuries later, Paul writes, "By faith even Sarah herself received ability to conceive, even beyond the proper time of life, since she considered Him faithful who had promised". (Hebrews 11:11)
Sarah's faith existed before she conceived of Isaac, not after, and she laughed because of that faith.
For a meaning of the name Isaac, NOBSE Study Bible Name List has Laughter and Jones' Dictionary of Old Testament Proper Names has Laughing. BDB Theological Dictionary proposes He Laugheth.