Is Tuesday named for a one-handed god?
1670s, "fall or sink into a muddy place," probably from a Scandinavian source, cf. Norwegian and Danish slumpe "fall upon," Swedish slumpa; perhaps ultimately of imitative origin. Related: Slumped; slumping.
The word "slump," or "slumped," has too coarse a sound to be used by a lady. [Eliza Leslie, "Miss Leslie's Behaviour Book," Philadelphia, 1839]Economic sense from 1888.
"act of slumping, slumping movement," 1850; "heavy decline in prices on the stock exchange," 1888, from slump (v.). Generalized by 1922 to "sharp decline in trade or business."