ABIGAIL (ăb'ĭ-gāl, Heb. ’ăviĝhayil, father is rejoicing)
2. Apparently a half-sister of David and of Zeruiah, the mother of Joab (1 Chron 2:16; 2 Sam 17:25). She was the daughter of Nahash and the mother of Amasa (17:25). It has been suggested that “daughter of Nahash” might be a text corruption, or that Nahash was another name for Jesse, or that Jesse married the widow of Nahash. Whether or not any of these surmises are true, Abigail and David had the same mother. Her husband was called Jether the Ishmaelite (1 Chron 2:16, 17 KJV) and Ithra the Israelite (2 Sam 17:25 KJV). Since Israelite or Ishmaelite might refer to the area in which he lived or to his race, both could be true. Abigail was the mother of Amasa who for a while was commander of David’s army (2 Sam 20:4).
International Standard Bible Encyclopedia (1915)
ab’-i-gal, ab’-i-gal (’abhighayil, or ’abhighal, three times, or ’abhughayil, once, or ’abhighayil, once; "father," or "cause of joy"):
(2) Sister of David and mother of Amasa, at one time commander of David’s army (1Ch 2:16,17; Abigal 2Sa 17:25). In the first passage she is called David’s sister, along with Zeruiah; while in the second she is called the "daughter of Nahash." Several explanations of this connection with Nahash have been suggested, any one of which would be sufficient to remove contradiction:
(1) That Nahash was another name of Jesse, as in Isa 14:29, mish-shoresh nachash yetse’ (Qimchi);
(2) That Nahash was the wife of Jesse and by him mother of Abigail, which is least probable;
(3) That Nahash, the father of Abigail and Zeruiah, having died, his widow became the wife of Jesse, and bore sons to him;
(4) That the text of 2Sa 17:25 has been corrupted, "daughter of Nahash" having crept into the text. At all events she was the sister of David by the same mother.