what's the meaning of "water off"?

[Romani]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #408 "Water Idioms", question 1

Although he had had a bad experience with his business, he treated it as water off a duck's back -- means: ..........

(a) of some importance
(b) of much importance
(c) of no importance
(d) of great importance

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #408 "Water Idioms", answer 1

Although he had had a bad experience with his business, he treated it as water off a duck's back -- means: of no importance.

Correct answer: (c) of no importance
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whats the meaning of water off?

Romani

[Mister Micawber]

.
as water [running] off a duck's back.
.
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Native English teacher at Mister Micawber's