What does Judges 9:18 mean?

Jotham finally delivers a direct condemnation on the people of Shechem. His brother, Abimelech, is the one who carried out the murders of Gideon's other sons, but Jotham is holding the people of Shechem responsible. They agreed with Abimelech's plan and provided money to hire hooligans for the deed (Judges 9:1–6). This resulted in some form of public execution, possibly even a ritual sacrifice.

By funding Abimelech's murders, the people are guilty of rising against Gideon's house instead of honoring the hero used by God to deliver them. They are responsible for the slaughter of Gideon's seventy sons. They have even made Abimelech king over them simply because he is a relative to some of them.

This condemnation ties to the fable which began Jotham's speech (Judges 9:7–15). That imagery casts Abimelech as a worthless, dangerous choice. In fact, Jotham chooses language here that is deliberately dismissive. He describes Abimelech as "the son of [his father's] female servant." Abimelech's mother was not Gideon's wife, but a concubine (Judges 8:31). In ancient middle easter culture, especially, this was no small insult. The implication is that Abimelech is "merely" the unwanted son of a servant—making him unworthy of being Shechem's king. This remark also has an obscure connection to Abimelech's eventual rival, whose name includes a reference to servanthood (Judges 9:26).