have a mind like a sieve
To be exceptionally forgetful or absentminded. I need to start getting more sleep at night, because lately I've had a mind like a sieve. John has a mind like a sieve—I had to leave him three separate reminders about the play tomorrow night!
have a mind/memory like a ˈsieve
(informal) forget things easily or quickly: I’m terribly sorry I didn’t remember your birthday — I’ve got a memory like a sieve.A sieve is a kitchen tool with small holes in it, used for separating solids from liquids or very small pieces of food from larger pieces.