This idiom means that outward appearances are not a reliable indicator of someone's true character or nature. Just because someone looks or acts like a monk doesn't mean they are actually a pious or virtuous person.
Here's why the phrasing is correct:
* "Habit" refers to the clothing worn by monks, which is often seen as a symbol of their religious devotion.
* "Monk" refers to a person who has dedicated their life to a religious order.
The idiom emphasizes that true character lies within, not in outward appearances.