Billy is the pet form of William, from the Germanic Willahelm, meaning "resolute protector" or "strong-willed warrior". It has been given as a full name in America for well over a century, from the outlaw Billy the Kid to the singer Billy Joel, and was a top 50 choice in the 1930s and 1940s. Billy ranked number 385 in 2000, drifted down through the 800s in the 2010s and stands at number 1068 in 2025.