1. [mid-19C+] the police.
2. [1910s+] (Aus./US) a blue uniform.
3. [1940s] any trousers.
4. [1940s] a police uniform.
5. [1960s+] (US prison) a prison uniform.
6. [1960s+] amphetamines.
7. [1970s] (US) a sailor’s trousers.
8. [1970s+] (US) blue jeans.
9. [1980s+] (Aus. prison) prison officers; thus used as adj. to refer to staff activity, e.g. blue talk, prison officer conversations etc.