Not to have an ounce of sense / To be very foolish
Literally 'not to have two fingers of forehead'. Describes someone who lacks basic intelligence or common sense. Can be used humorously.
How could he do that? He doesn't have an ounce of sense.
You only need an ounce of sense to see that's a bad idea.
—Why did she do it like that? —She doesn't have an ounce of sense.