The idiom “like water off a duck’s back” means that something has no effect on a person or situation. It implies that criticisms, insults, or warnings are being ignored and have no impact on the person being addressed
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Here’s a more detailed explanation:

Origin:
The saying refers to the way water slides off a duck’s feathers due to their oily and waterproof nature. Ducks are constantly preening to maintain this waterproof state.

Meaning:
When something is “like water off a duck’s back,” it means it has no impact, leaves no trace, or doesn’t bother the person. It’s similar to saying someone has “thick skin” or “roll with the punches”.
Usage:
You might use this phrase when describing someone who is not affected by criticism or who doesn’t seem to listen to advice. For example, “I tried to warn him, but it was like water off a duck’s back; he just wouldn’t listen”.
Related Idioms:
Other idioms that convey a similar meaning include “fall on deaf ears,” “roll off the tongue,” and “bounce right off”