2 Samuel 17:25

And Absalom made Amasa captain of the host instead of Joab,
&c.] Who was along with David:

which Amasa [was] a man's son whose name [was] Ithra, an Israelite;
he is called Jether the Ishmaelite, ( 1 Chronicles 2:17 ) ; either because he was an Ishmaelite by birth, and being proselyted to the Jewish religion, was called an Israelite; or rather he was an Israelite by birth, and having sojourned in the land of Ishmael some time, was called an Ishmaelite, as Uriah the Hittite, and others; and when Amasa is called a "man's son", it means a great man, as Kimchi observes:

that went in to Abigail, the daughter of Nahash, sister to Zeruiah,
Joab's mother:
so that Joab and Amasa, the generals of David and Absalom, were own cousins, sisters' children, and David uncle to them both; for Abigail and Zeruiah were David's sisters, ( 1 Chronicles 2:16 ) ; children of Jesse, whose name is here called Nahash, having two names, or this was his surname; though others think Nahash is the name of a woman, the wife of Jesse, and that these two sisters had two mothers, but one father; it seems by this expression, "went in to", as if Amasa was not begotten in the conjugal state, but was illegitimate.