a mind like a sieve
The tendency to be exceptionally forgetful or absentminded; a poor memory. 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!
a mind like a sieve
or a brain like a sieve
If you have a mind like a sieve or a brain like a sieve, you have a bad memory and often forget things. He's lost his keys again — he's got a mind like a sieve, that boy.