birdbrain; scatterbrain; forgetful person
Colloquial and mildly self-deprecating or teasing term for someone who is very forgetful or has a poor memory, likening them to a bird's supposedly small brain.
You forgot our plans again? You're such a birdbrain.
I'm so forgetful that I have to write things down right away or I'll forget.
Compound of 鳥 (bird) and 頭 (head), literally 'bird head'. The exact origin is uncertain, but it likely comes from the folk belief that birds have small brains and thus poor memory.