John Gill Commentary 1 Chronicles 2:17

"And Abigail bare Amasa; and the father of Amasa was Jether the Ishmaelite." — 1 Chronicles 2:17 (ASV)

And Abigail bare Amasa
Who was Absalom's general, afterwards reconciled to David, and designed to be made general of his army, but was slain by Joab, see (2 Samuel 17:25) (19:13) (20:10) ,

and the father of Amasa was Jether the Ishmaelite ;
he is called an Israelite, (2 Samuel 17:25) , and so in the Targum here, he being either a proselyte, or else he was an Israelite by birth, but called an Ishmaelite, because he had dwelt among the Ishmaelites some time, as Obededom is called the Gittite for the like reason; so Jarchi and Kimchi interpret it.