Green’s Dictionary of Slang

an excl. indicative of a sudden action; also used as adv.

1894		Coburg Leader (Vic.) 10 Nov. 2/5: The Parkview going down ‘wollop’ second innings — 5 wickets for 12 runs .
1896		Pink & LeBrunn [perf. Marie Lloyd] ‘Chance Your Luck’ 🎵 [Y]our annoyance do not show / Don’t go wallop off your head.
1907		Godfrey & Victoria [perf. Vesta Victoria] Mary Queen of Scots 🎵 I came down wallop on poor Charles the First.
1914		Truth (Melbourne) 10 Jan. 11/2: A snappy right to the chin that put him down wallop.
1917		Union Jack 5 May 17: Down he threw himself – wallop!
1938		N. Lindsay Age Of Consent 85: Came down wallop on the flower-bed.
1984		A. Sayle Train to Hell 58: I go for a little run, and then wallop! I’m lying on a [...] floor with a bloody great knife sticking out of my stomach.
1990		in D. Campbell That Was Business, This Is Personal 20: As we get outside — wallop. ‘Robert King you are under arrest.’.
1997		(con. 1950s) J. Byrne Slab Boys [film script] 59: Ah, but that’s where yur wrong, old bean ... ye only get the wan shot then it’s ... wallop, you deid.