Definition & Meaning of "water off a duck's back"in English

What is the origin of the idiom "water off a duck's back" and when to use it?

The idiom "water off a duck's back" finds its origins in the observation that water rolls off a duck's waterproof feathers without penetrating or affecting the bird. This metaphorical expression conveys the idea that criticism, insults, or negative comments have no impact or effect on a person, similar to how water doesn't stick to a duck's feathers. It is used to suggest resilience and encourage the ability to remain unaffected by adverse situations or remarks.