Quick definitions from WordNet (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)

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▸ Also see billy