Genesis 21:9 New English Translation

⁹But Sarah noticed^[a] the son of Hagar the Egyptian—the son whom Hagar had borne to Abraham—mocking.^[b]

Genesis 21:9 tn Heb “saw.”
Genesis 21:9 tn The Piel participle used here is from the same root as the name “Isaac.” In the Piel stem the verb means “to jest; to make sport of; to play with,” not simply “to laugh,” which is the meaning of the verb in the Qal stem. What exactly Ishmael was doing is not clear. Interpreters have generally concluded that the boy was either (1) mocking Isaac (cf. NASB, NIV, NLT) or (2) merely playing with Isaac as if on equal footing (cf. NAB, NRSV). In either case Sarah saw it as a threat. The same participial form was used in Gen 19:14 to describe how some in Lot’s family viewed his attempt to warn them of impending doom. It also appears later in Gen 39:14, 17, where Potiphar accuses Joseph of mocking them. sn Mocking. Here Sarah interprets Ishmael’s actions as being sinister. Ishmael probably did not take the younger child seriously and Sarah saw this as a threat to Isaac. Paul in Gal 4:29 says that Ishmael persecuted Isaac. He uses a Greek word that can mean “to put to flight; to chase away; to pursue” and may be drawing on a rabbinic interpretation of the passage. In Paul’s analogical application of the passage, he points out that once the promised child Isaac (symbolizing Christ as the fulfillment of God’s promise) has come, there is no room left for the slave woman and her son (who symbolize the Mosaic law).

New English Translation (NET)