These terms usually appear together to refer to that part of the wreckage of a ship and its cargo found floating on the water or washed ashore. The phrase "flotsam and jetsam" now has an extended meaning of "useless trifles," "odds and ends": "The attic was filled with the flotsam and jetsam of many years." Although the words are now inseparable, in law they have distinct meanings. Flotsam (from an Old English word meaning "to float") is that part of a wreck that is floating on the surface; jetsam (from a Latin term meaning "to throw") refers to cargo tossed overboard to lighten a ship or improve its seaworthiness. Centuries ago, the flotsam of a wreck belonged to the king, jetsam to the lord of the manor off whose property the wreck occurred.
Dictionary of problem words and expressions. Harry Shaw. 1975.