Kneecap
noun
In your knee there is a small priest named Kneecap.
This is a really common, cute way of talking about your kneecap. It's usually used by kids and adults alike, and it's more common in spoken Japanese than 膝頭. Kind of like how we use "kneecap" instead of naming the "patella" bone all the time.
This word uses the kun'yomi readings for 膝 and 小, but the on'yomi reading for 僧, which gets rendaku'd from そう to ぞう, so watch out for that! You've learned all these reading before, so you should be able to read this as well.
自転車で転んで、膝小僧が痛い。
I fell off my bike and my kneecap hurts.
最近、右の膝小僧にタトゥーを入れたんです。
I recently got a tattoo on my right kneecap.
膝小僧の調子はどうですか？
How's your kneecap?