Definitions of billy:

• noun:   male goat
• noun:   a short stout club used primarily by policemen
• name:  A male given name (very common: 1 in 403 males; popularity rank in the U.S.: #73)
• name:  A female given name (rare: 1 in 25000 females; popularity rank in the U.S.: #1481)
• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #8671)