Flotsam is stuff that just floats about, jetsam is stuff that has been jettisoned from a boat, i.e. thrown over board. That's because it comes from the Latin iacare meaning to throw. That's why, when Julius Caesar crossed the Rubicon and led his troops into Italy, he said Alea iacta est. It means The dice have been thrown.
If you throw out liquid or vapour, it forms a jet of water or steam. (If you throw out a wooden structure from the shore, it forms a jetty). Newtonian physics insists that if you throw out a jet in one direction you will be pushed in the other. This is the principle of the jet engine.
The first commercial jet liner was the de Havilland Comet, which first flew in 1949, but didn't start doing business until 1952. It's therefore a trifle surprising that the jet set was first recorded in 1949. However, that's because the jet set was originally just young people who lived fast. As fast as jets.
The inevitable result of real, literal jet-setting is jet lag, which was first recorded in 1965. It popped up in the New York Herald Tribune and is described thuslyly:
Jet lag strikes suddenly. The victim disembarks from the..plane feeling gay as a sprite, dashes through customs, checks into home or a hotel, .. greets friends and in the course of the next few hours falls into a light coma.
Which is why I was up at 5am this morning. Australia was wonderful.