Английский Этимологический словарь - slump (v.)

- 1677, "fall or sink into a muddy place," probably from a Scand. source, cf. Norw. and Dan. slumpe "fall upon," perhaps ultimately of imitative origin. The noun meaning "heavy decline in prices on the stock exchange" is from 1888; generalized to "sharp decline in trade or business" 1922.