c. 1600, from Anglo-French floteson, from Old French flotaison "a floating" (Modern French flottaison), from floter "to float, set afloat" (of Germanic origin and cognate with float) + -aison, from Latin -ation(em). Spelled flotsen in English till mid-19c. when it altered, perhaps under influence of many English words in -some. Folk-etymologized in dialect as floatsome.
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. Flotsam and jetsam figuratively for "odds and ends" is attested by 1861.
updated on April 21, 2017