kneecap; patella
Common, slightly informal or child-friendly word for the kneecap. The more formal medical term is 膝蓋骨 (しつがいこつ).
I fell and scraped my kneecap.
When I was a child, I often put a bandage on my kneecap.
Compound of 膝 (ひざ, 'knee') and 小僧 (こぞう, 'young boy' or 'apprentice'), likely from a playful or metaphorical association of the protruding kneecap with a small bald-headed figure. The exact origin is uncertain.