Green’s Dictionary of Slang

1. a hard blow; a fatal wound.

1821–6		‘Bill Truck’ Man o’ War’s Man (1843) 180: He’s received a slapper smack in the bosom.
1844		Era (London) 21 Jan. 11/3: Charley at length hit him with two slappers on the mouth.

2. (N.Z. prison) a sticky rubbber band, used in the transfer of contraband.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 169/2: slapper n. a small, sticky rubber band [...] these are popular in prison because inmates may attach an item of contraband [...] to the slapper and throw it across the yard to stick to the cell window of a fellow inmate.