Abimelech’s mother was not reckoned among the wives, being, probably, one of the Canaanite population in Shechem Judges 9:28 : neither was Abimelech himself reckoned with the 70 other sons of Jerubbaal (Judges 9:24. Compare Judges 11:1-2).
Related Commentaries of Judges 8:31
Judges 8:31
31 And his concubine that was in Shechem, she also bare him a son, whose name he calledi Abimelech.