The story
Probably of North Germanic origin: compare Danish slumpe (“to stumble upon by chance”), Norwegian slumpe (“happen by chance”), Norwegian slumpa (“happen by chance”), Swedish slumpa (“randomize; to sell off”), Swedish slump (“chance, randomness, happenstance”). Compare also German schlumpen (“to trail; draggle; be sloppy”), dialectal Dutch slompen (“to walk clumsily”).
Etymology from Wiktionary, CC BY-SA 4.0.