Green’s Dictionary of Slang

when falling or hitting, hard and suddenly; also as n., a fall or dive.

1838		D. Crockett in Meine Crockett Almanacks (1955) 121: I went chewallop into the water.
1888		Hoppe Eng.-Deutsches Supplement-Lex. 215: It means ‘flat down,’ and is a strong expression. If a woman, for ex., falls head over heels and flat to the ground, they say, ‘she fell chewallop’ [F&H].
1960, 1975		Wentworth & Flexner DAS 97/1: chewallop, chewalloper n. A fall or a dive that makes a big splash or flopping noise.