The sad fact about ignorance is that it tends to creep back up on you. I know, for instance, that I once knew exactly what flotsam and jetsam was. I know this because I know exactly where I learned of it. It was in a book one of my early grade school teachers read to us, and I believe we may have even done some kind of science project on it.
Because of Pagoo, the hermit crab, we learned a lot about tidepool life. My sense is that flotsam and jetsam are what float in and out with the tide. Debris? But am I even right about that? And if so, which is which?
Well, I would have to reread the book to know how flotsam or jetsam ended up in Pagoo's tidepool. Now that I think about it, maybe it was just plankton. Anyway, much to my relief, flotsam and jetsam are indeed debris, not microscopic sealife. They are terms in maritime law which define different kinds of cargo that end up in the sea. Flotsam, you guessed it, is cargo or wreckage that remains floating on the sea after the shipwrecked ship has sunk. Jetsam is cargo or equipment that has been thrown from a ship in distress to lighten its load, and by extension cargo that has ended up washed up on shore. There is even another term, lagan, for whatever sinks to the ocean floor. (Sometimes, though this word means cargo thrown overboard that has been attached to a buoy with the hope of being able to retrieve it later.)
Flotsam comes from Anglo-French floteson, and back from there to Old French flotaison, "a floating". As for jetsam, as I was writing this up, I wondered if it was related to the word "jettison", but decided that was too unlikely. Wrong. Jettison, the act of throwing overboard, was actually restored to this original meaning after it had drifted into being jetsam, which had the vague sense of things cast overboard. The sequence of jetsam goes back through "jottsome" to Middle English jetteson to Anglo French getteson, and finally to Old French gettaison-- "a throwing".
A-floatin' and a-throwin' and a-sinkin' is about the size of it all.
But the figurative use of the phrase to mean 'odds and ends' comes up as early as 1861. And even before this, Sir Walter Scott, though retaining more of the original meaning, wrote in his Diary of 1848,
"The goods and chattles of the inhabitants are all said to savour of Flotsome and Jetsome."
Ouch, Sir Walter.