The criminal underworld
During the 1920s the criminal element became more visible and its language entered the mainstream, describing both criminals and the police. Police officers were bulls unless they were Irish in which case they were called mulligans, and attempting to elude them was called being on the lam. Detectives were dicks. Lying to an officer (or anyone else) was called feeding a line, as in; Mulligan found my blue nose but I fed him a line and he let me blouse off. Guns were gats, rods, and in the case of the machine gun, a chopper. The criminals were hoods, torpedoes, and thugs, the crimes they pulled off capers.
The doormen guarding illegal speakeasies, and other dens of iniquity such as gambling rooms, bookie joints, and brothels, were usually large men called baby grands. Most were hard-boiled bimbos. Irish doormen were usually called harps, and if one wished to engage them in fisticuffs the more derogatory mick. When someone was arrested he was pinched; if the evidence against him was false he was framed, which made him a fall guy. Killing someone was bumping him off, and robbing someone or something was knocking him or it over, as in, he knocked over the drum and now he’s on the lam.