Green’s Dictionary of Slang

locust n.¹

1. a policeman.

1833		Poor man’s Guardian 25 May 3/1: The blue-coated locusts [...] came down on them without ceremony.
1865		G.A. Sala My Diary in America II 211: The New York policeman wears a handsome uniform. At his side hangs a club or bludgeon [...]. This club is made of ‘locust wood,’ [...] and by rowdies the policeman is often generically called (with the addition of a frightful expletive) a ‘locust’.

2. a billy club or stick.

1852		London Dly News 10 Aug. 2/6: [from N.Y. Herald] The police generally were provided with a locust club.
1855		Broadway Belle (NY) 24 Sept. n.p.: The ghost of our efficient Chief [of Police] appears with ‘locust’ in hand.
1866		Night Side of N.Y. 38: The roundsman gives a certain number of raps with his ‘locust’ upon the sidewalk flags.
1868		H.L. Williams Black-Eyed Beauty 11: She heaped him with abuse from the glaze of his cap to his boots, from locust to brass buttons. He tried to arrest her.
1882		J.D. McCabe N.Y. 383: Give them the locusts, men [DA].
1891		J.A. Riis How the Other Half Lives 73: Half a dozen of the group made a sudden dash, and with shoulders humped above their heads to shield their skulls against the dreaded locust broke for the door.
1894		Mirror of Life 20 Jan. 6/4: [B]y a deft use of the locust [he] warmed up the seats of the youngsters.
1896		‘Frederick Benton Williams’ (H.E. Hamblen) On Many Seas 397: Tom came down lustily on top of his ornamental cap with a good locust belaying-pin, and I grabbed him by the throat, planting an upper cut under his jaw.
1908		‘O. Henry’ ‘Squaring the Circle’ in Voice of the City (1915) 130: A locust club tickled him in the ribs.
1915		S. Ford Shorty McCabe on the Job 60: In that section of the White Light district too you can call up plenty of help by a few taps from the locust.
1926		N. Klein ‘Hobo Lingo’ in AS I:12 652: Locust — police club.
1933		(con. 1890) H. Asbury Barbary Coast 160: Not until about 1890 did the San Francisco police learn that [...] the best cure for hoodlumism is the frequent application of locust or hickory to the hoodlum’s skull.
1949		Monteleone Criminal Sl. (rev. edn).

In phrases

dose of locust (n.)

a beating.

1880		Memphis Daily Appeal (TN) 13 Feb. 4/4: If the police force can get hold of him, the administration of a wholesome dose of locust [...] would not be looked upon with any great horror.
a.1909		press cutting in J. Ware Passing Eng. of the Victorian Era 116/1: Mullaley, smarting under the pain of the wound, gave Mr Supple a dose of locust, which induced him to accompany the officer to court.

oil of locust (n.)

a beating.

1914

Sun (N.Y.) 3 Dec. 6/2: The best Medicine for Gangsters [...] A generous dose of oil of locust [...] The impact of the club on their skulls drives them into gooseflesh. out with the nightstick and down with the gangs.