Definition (behavior)
This phrase is used to indicate that you believe someone is acting unwisely or foolishly.
"He invested all his savings in that startup company." "More fool him, the company was so risky!"
"She gave the salesman money knowing he was unreliable." "More fool her, she was warned not to do that!"
"They signed the contract without checking it carefully." "More fool them, it will cost them later."