Buddhist Monk
Shaved Head, Boy
noun
A monk master is still a monk, and this is used for a Buddhist monk specifically. Keep in mind this isn't a word you'd use to address monks directly though, so don't use it when you're speaking to them!
This word also is used for describing a shaved head (you can probably guess why). That might be why it can also be a casual way of addressing a young boy, since in the old days a shaved head was the default hairstyle for boys in Japan.
The 坊 reads like you'd expect. Unfortunately 主 is an exception and read ず. Just think of a zoo that holds Buddhist monks in captivity. How sad for those Buddhist monks.
坊主になる前に頭を坊主にした。
Before becoming a Buddhist priest, I shaved my head.
明日、バリカンで坊主にする。
Tomorrow, I'll shave my head with clippers.
息子よ。現実的になれ。いい加減俺の後を継いで坊主になったらどうなんだ？
Hey, son. You have to be realistic. Isn’t it about time you followed in my footsteps and became a Buddhist priest?