The saying is said to originate in the 1800s. It comes from the literal characteristic that ducks feathers have for resisting water. If you have ever observed a duck, you may have noticed that no matter how often the duck dives under the water, it comes back up looking quite dry and water rolls off of them.

Male Mallard.

Northern shoveler. (male)

Northern shoveler. (female)

Shellduck.

The phrase “A nice day for ducks” is a pejorative expression used to describe a rainy day that can only be seen as being positive by those living things, like ducks, that enjoy a watery environment.