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c.1600, from Anglo-French floteson, from Old French flotaison "a floating," from floter "to float" (of Germanic origin; see float) + -aison, from Latin -ation(em). Spelled flotsen till mid-19c. when it altered, perhaps under influence of many English words in -some.
In British law, flotsam are goods found floating on the sea as a consequence of a shipwreck or action of wind or waves; jetsam are things cast out of a ship in danger of being wrecked, and afterward washed ashore, or things cast ashore by the sailors. Whatever sinks is lagan. Figurative use for "odds and ends" attested by 1861.