noun
memory like a sieve; in one ear and out the other
Idiomatic expression for someone who quickly forgets what they hear; often used self-deprecatingly or to tease.
彼はざる耳で、言ったことをすぐ忘れる。
He has a memory like a sieve and forgets what you tell him right away.
私、ざる耳だから、もう一度言ってくれる？
I have a memory like a sieve, so could you say that again?
ざる is a sieve or colander; ざる耳 literally means 'sieve ear', figuratively describing a leaky memory.
忘れっぽい is a more general adjective for 'forgetful', while ざる耳 specifically refers to forgetting things one hears.
Compound of ざる (sieve) and 耳 (ear), literally 'sieve ear'. The image is of information leaking out of the ear like water through a sieve.