billy · noun1
Of obscure origin. Perhaps a variant of bully (“companion, mate, comrade”). Compare Scots billie (“a comrade; companion”). Compare also Middle Low German billig (“equitable, reasonable, lawful, fitting, according to natural law, just”).
billy · noun2
Uncertain, but probably extracted from Scots billypot (“a type of cooking pot”), perhaps originally from the name Billy, a diminutive of William. The condom sense from the E-Rotic song Willy, Use a Billy... Boy, referring in turn to Billy Boy, a German brand of condoms.
Billy · proper noun3
Etymology from Wiktionary, CC BY-SA 4.0.