A billy is an informal, old-fashioned word for a club or stick, especially one carried by police officers. In the 1800s and early 1900s, police officers in America and Britain carried wooden clubs called billy clubs or nightsticks for protection. You might read about a detective tapping his billy against his palm while walking a nighttime beat in old mystery stories.

Billy can also refer to a male goat, a kettle for boiling water over a fire, or a nickname for “William.”