young priest; scamp
Literally a young Buddhist priest or acolyte. Also used as a teasing or affectionate term for a mischievous boy, similar to 'little rascal'.
寺で小坊主たちが掃除をしていた。
The young priests were cleaning at the temple.
うちの小坊主がまたいたずらをした。
Our little scamp got into mischief again.
Compound of 小 (small, young) and 坊主 (Buddhist priest). The extended meaning 'scamp' comes from the image of a young, playful acolyte.