What does Judges 9:17 mean?

Rather than simply telling the people of Shechem that Abimelech would be a worthless, dangerous king, Jotham began with a parable (Judges 9:7–15). Now, he finally says directly what his fable presented in poetry. Jotham doesn't bother to call out his brother Abimelech for his atrocities (Judges 9:1–6)—Abimelech is treated as one beyond hope of repentance. Rather, Jotham scolds the people of Shechem for participating in the slaughter of all his other brothers.

His father, Gideon, risked his own life to fight the Midianites (Judges 7:19–23) to deliver the people from oppressive annual raids (Judges 6:1–5). These attacks left the country in poverty and ruin. Jotham does not go into detail about how it was the Lord at work through Gideon that accomplished this (Judges 7:2). He simply describes why Gideon's memory and household should have been honored after his death. Instead, as Jotham will show, the opposite has happened.

The point of Jotham's speech is that Shechem has "settled" for a dangerous, worthless man as their king, simply because he is not one of Gideon's other sons. That will come back to haunt them.