Like water off a duck's back

The phrase "Like water off a duck's back" means that something does not affect or bother someone at all. It suggests that criticism, insults, or negative events have no impact on the person and they are able to easily let them slide off or ignore them. Just as water rolls off a duck's waterproof feathers, the person remains unaffected and unperturbed by the negativity, maintaining a calm and resilient attitude.