Charles Ellicott Commentary 2 Samuel 17:25

"And Absalom set Amasa over the host instead of Joab. Now Amasa was the son of a man, whose name was Ithra the Israelite, that went in to Abigal the daughter of Nahash, sister to Zeruiah, Joab`s mother." — 2 Samuel 17:25 (ASV)

Amasa. —Joab having adhered to David and gone away with him, Absalom chose his cousin to succeed him as commander-in-chief.

Ithra an Israelite. —Called in 1 Chronicles 2:17 Jether the Ishmeelite. Jether and Ithra are merely different forms of the same name; but Israelite is probably an error for Ishmeelite. The Septuagint has, in the Alexandrian copy, Ishmaelite, and in the Vatican, Jezreelite.

Abigail the daughter of Nahash. —Since this Abigail is said to be “sister to Zeruiah,” and in 1 Chronicles 2:16 both Abigail and Zeruiah are said to be the sisters of Jesse’s sons, it follows, either that sister is used in the sense of half-sister, or else that Nahash, usually a man’s name, was the name of Jesse’s wife. It is impossible to decide certainly. The Jewish tradition that Nahash is another name for Jesse has no support.