Green’s Dictionary of Slang

billy n.4

1. [mid-19C] a short iron crowbar, used by criminals.

2. [mid–late 19C] (UK Und.) stolen metal.

3. [mid-19C+] (orig. US, also bill, billie, billy stick) a policeman’s wooden club (orig. untanned cowhide, covered in wool).

4. [mid-19C+] a form of blackjack, usu. ‘loaded’ with lead.

5. [1960s] (US) a police officer; the police as a group.

6. [2000s] (US prison) a white man.

In compounds

billy boy (n.)

[1960s] (US) a policeman.

billy-hunting (n.)

[mid-19C] trading in old (poss. stolen) metal; thus billy-hunter, billy-prigger, a trader in such metal.

billy man (n.)

[1960s] a policeman.