What does Judges 9:40 mean?

This is the anticlimactic explanation of the battle set up over the last several verses (Judges 9:30–39). When the fight happens, it is not much of a fight. Gaal's bold words to the leaders of Shechem against Abimelech (Judges 9:28–29) have led them to do battle with Abimelech and his forces. The leaders who initially put Abimelech in power (Judges 9:1–6) no longer want him to rule (Judges 9:22–25). They are in open rebellion against him, led by outspoken Gaal (Judges 9:26).

Despite Gaal's bravado and the rebellion of Shechem's leaders, Abimelech and his men are too powerful for them. Gaal sees which way things will go with the battle, so he runs for his life and escapes. Others are not so fortunate. Many of the leaders of the city fall near the city's eastern gate. The Hebrew root word describing those who fell is, most often translated as "kill, slay, pierced, or profaned." It's possible Abimelech made a point of maiming city leaders instead of killing them. It's also possible this comment includes both those wounded and killed outright, as when the term "casualties" is used in modern English.

Regardless of such details, Abimelech has defeated his former patrons. He will more thoroughly punish the rest of the city on the following day (Judges 9:42–45).