Green’s Dictionary of Slang

law n.

1. [mid–late 16C] (UK Und.) a type of criminal activity [OF lei; ult. Lat. legem, law; synon. with lay n.3 (1)].

2. [18C+] (orig. US) constr. with the, the police.

3. [1910s+] with the article deliberately omitted, a police officer; occas. a private detective.

4. in attrib. use of sense 3.

5. [1940s+] (US prison) a prison warder.

In compounds

law-car (n.)

[1970s] a police car.

lawman (n.) (also lawboy)

[1960s+] a law-enforcement officer.

law station (n.)

[1950s] a police station.

In phrases

give law (v.) [sporting jargon give law, to give a hunted animal a chance of escape]

[late 18C–mid-19C] to give someone a chance.