Green’s Dictionary of Slang

redcap n.

1. [1900s–60s] (also red one) the penis.

2. [1910s+] (orig. milit., also cherry-nob) a military police officer.

3. [1910s+] (US) a railway porter; an airline porter.

4. [1980s+] (drugs) in pl., generic for crack cocaine [the red-capped vial in which the drug is often sold].