Green’s Dictionary of Slang

whister-clister n.

also whisterpoop
[dial. whister, whisper + SE clyster, an enema]

a blow on the ear.

[UK]Shakespeare London Prodigal B3: If it were not vor shame, chee would a giuen thee zutch a whister poope vnder the eare.
[UK]‘Exmoor Courtship’ in Gentleman’s Mag. June 298/1: Vurst ha geed ma a whisterpoop under tha year [Ibid.] 299/2: Add! chell gi’ en zutch a zwop! — chell gi ’en a whapper, and a wherret and a whisterpoop too.
[UK]Grose Provincial Gloss. n.p.: Whisterpoop. A back handed blow [...] Exm[oor].
[Scot]W. Scott Heart of Mid-Lothian (1883) 328: I redd thee keep hand off her, or I’se lend thee a whisterpoop.
[UK]Farmer & Henley Sl. and Its Analogues VII 340/1: Whister-clister (Whister-snefet, Whister-snivet, Whister-twister, or Whister-poop) [...] a thumping blow; spec. a back-handed blow.