a foolish action, remark etc is stupid and shows that someone is not thinking sensibly [= silly]:
I've never heard anything so foolish in all my life.
It would be foolish to ignore his advice.
be foolish enough to do something
I was foolish enough to believe him.
a foolish person behaves in a silly way or looks silly [= stupid]:
I was young and foolish at the time.
a foolish grin
He'd been made to look foolish.
She foolishly agreed to go with them.
—foolishness noun [uncountable]