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).

His concubine - A lawful but secondary wife, whose children could not inherit.
Whose name he called Abimelech - That is, my father is king, or my father hath reigned. This name was doubtless given by the mother, and so it should be understood here; she wished to raise her son to the supreme government, and therefore gave him a name which might serve to stimulate him to seek that which she hoped he should enjoy in his father's right. See the following chapter, Judges 9 (note).

And his concubine that was in Shechem,.... Which was not an harlot, but a secondary or half wife; such were generally taken from handmaids, and of the meaner sort, and were not in such esteem as proper wives, had not the management of household affairs, only a share in the bed, and their children did not inherit. This concubine of Gideon's seems not to have been taken into his house at all, but lived at Shechem, perhaps in her father's house, and here Gideon met with her when he went to Shechem as a judge to try causes; her name, according to Josephus (l), was Druma:
she also bare him a son; as his other wives did; perhaps all the children he had were sons, and this was one over and above the seventy, and not to be reckoned into that number:
whose name he called Abimelech: which signifies, "my father a king"; which he gave him either in memory of the offer made him to be king of Israel, or through foresight of what this son of his would be; or he might be moved to it by the mother from pride and vanity, and which name might afterwards inspire the young man to be made a king, as he was; and the account given of his name is because of the narrative of him in the following chapter.
(l) Antiqu. l. 5. c. 7. sect. 1.

Shechem - She dwelt there, and he often came thither, either to execute judgment, or upon other occasions. Abimelech - That is, my father the king; so he called him, probably, to gratify his concubine, who desired it either out of pride, or design.