An unlikely chance occurrence, or a parasitic flatworm, or the flat end of an anchor arm.
Multiple etymologies: the 'chance' meaning comes from billiards slang (19th century) for an accidentally successful shot. The parasitic worm sense comes from Old English floc meaning 'flatfish'. The anchor part comes from its resemblance to a flounder's flat shape.
This word perfectly demonstrates how English allows completely unrelated meanings to share the same form - pure coincidence brought together lucky accidents, parasitic worms, and anchor parts under one spelling. It's a linguistic 'fluke' that shows the playful chaos of English etymology.
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