noun
Children's word for 'head'; used affectionately or playfully, often by or to small children.
おつむをぶつけちゃったの？
Did you bump your little head?
noun
Playful or childish way to refer to intelligence or mental ability; often used in light teasing.
あの子はおつむがいいね。
That kid is smart, isn't he?
Likely a sound-symbolic or baby-talk formation; the exact origin is uncertain.