meaning of "keeps his nose clean"

[Min Kin Go]

English Idioms and Expressions, Elementary Level

ESL/EFL Test #12 "Just keep it", question 6

The main thing that you have to admire him for is that he doesn't get into trouble and keeps his nose clean.

(a) blows his nose a lot
(b) behaves legally
(c) sneezes often
(d) breathes easily

English Idioms and Expressions, Elementary Level

ESL/EFL Test #12 "Just keep it", answer 6

The main thing that you have to admire him for is that he doesn't get into trouble and behaves legally.

Correct answer: (b) behaves legally
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Is there any story or explanation can explain why to keep one's nose clean is a kind of behave legally?

Min_Kin Go

[Mister Micawber]

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Nothing beyond the obvious, I think.
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Native English teacher at Mister Micawber's