willie
n. countable A1 Beginner US //ˈwɪɫi// UK //wˈɪli// willie
n. a common short name for a man named William. It is a friendly or casual way to address someone.
n. a diminutive form of the masculine given name William. Primarily used as a familiar or informal hypocorism.
My uncle Willie lives in a small house by the sea.
The local mechanic, known to everyone as Willie, has fixed cars in this neighborhood for thirty years.
While his legal documents all bore the name William, he insisted that even his business associates address him as Willie to maintain a personable atmosphere.
From Will + -ie.
Usage
As a proper noun, it is almost always capitalized.