What does the saying Like water off a back mean?

The saying is actually "like water off a duck's back". It means that something has no effect on someone, that they are unaffected by it.

Here's why:

* Duck feathers: Ducks have special feathers that are coated in oil, making them waterproof.

* Water rolling off: When water hits a duck's feathers, it rolls right off, leaving the duck dry.

So, the saying is comparing someone's ability to deal with something negative or challenging to a duck's ability to shrug off water. They are unaffected, just like water rolling off a duck's back.