Green’s Dictionary of Slang

1. (Scot., also Billy boy) a Protestant.

1935		G. Blake Shipbuilders (1954) 149: Did it ever occur to you that Cosh Tanelli’s a Pape and the old man on the bench is a Billy Boy, blue and orange as they make them?
1994		I. Welsh ‘The Two Philosophers’ Acid House 116: If the handshake is right he [i.e. a policeman] will let the Billy go and slap the Tim around a bit.
2004		C. Anderson [bk title] The Billy Boy: The Life and Death of LVF Leader Billy Wright.
2013		T. Black Ringer [ebook] n.p.: As I turn to the barman I can see the good old Billy Boy is nodding in approval.

2. see King Billy n.