Green’s Dictionary of Slang

lily law n.

(orig. US gay) the police.

1949		‘Swasarnt Nerf’ et al. Gay Girl’s Guide 12: lily: MBS [Male Bobby-Soxer] term for policeman. often lily law (espcially in plural).
1961		J. Kirkwood There Must Be a Pony! 243: ‘Don’t you want the chicken?’ ‘Not with Lilly Law lookin’ on!’ she said, indicating the cops.
1962		R. Hauser Homosexual Society Appendix 3, 167: Lilly, the law.
1972		B. Rodgers Queens’ Vernacular 125: Lilly (Law) the police [...] Miss Lily.
1973		(con. 1927–30) E. Williams Emlyn 27: Then there’s my antique dealer friend, likes chickens and since anything underage is dangereux I warned him to prendre garde a Lily Law.
1980		Maledicta IV:2 (Winter) 230: Lillie Langtry (1852–1929) [...] gave us the name Lily meaning both prostitute and policeman (Lily Law).
1999		G. Seal Lingo 116: lily is the police.
2003		K. Cage Gayle 80/1: Lily n. policeman (abbr.) of Lily Law and her silver bangles.
2010		(con. 1960s) W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 72: In the early 1960s in New Zealand Vivian Vice, Nelly-law, Dolly Handbag, Alice, Dora-D, Hilda-Handcuff, Lily-lunchbox, Jennifer-Justice, Hilda Box-rot, Petunia Pig, Tilly Tight-twat, and Cherie Cunstable, were familiar.
2019		P. Baker Fabulosa 294/1: Lily (Law), lilly the police.

In compounds

lily pad (n.)

(Polari) a police station.

2019

P. Baker Fabulosa 294/1: lily pad police station.