The answer appears to be a pun on the word “woman” (’išša). The word for fire is ’ēš. Thus the words for woman and fire form a wordplay. Abimelech’s attempt to burn down the tower of Thebez with fire (’ēš) “backfires” when a woman (’išša) drops a stone on his head.
53 And a certain woman threw an upper millstone on Abimelech’s head and crushed his skull.