[SPOILER]For example jettison means to remove cargo or other items to lighten weight in planes during a low fuel emergency. Jetsam is the older word that was used for naval vessels.
Last ditch refers to literally the last line of defense in a military setting. I also had heard about hunters employing bush beaters long ago. Even today hunters use bird dogs to flush quail out of brush. Same idea.
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The others I found very interesting. Learned a lot. http://www.rd.com/slideshows/8-amusing-stories-behind-common-expressions/
Why do we say that someone who is fired gets the sack?
Why do we say that someone who has appropriated someone else’s ideas or future remarks has stolen thunder from the victim?
What is the difference between a ‘kit’ and a ‘caboodle’?
Why is a final effort called last ditch?
Why is mincing around a subject called beating around the bush?
What is the difference between ‘flotsam’ and ‘jetsam’?
Does anyone ever engage in low jinks? And what’s a jink?
Those might be correct explanations; they might not; or they might be a mixture. That site gives no references, so it’s hard to tell without independent investigation.
Quickie answer: Jetsam is stuff that’s been deliberately tossed off a ship, to lighten it in emergency conditions. (Which is one reason I tend to think that jettison predates air travel, BTW, unlike the origin you offer in spoilers.) Flotsam is stuff that floated to the surface after a ship sinks.
Often impossible to differentiate when encountered at sea, or washed up on the shore, AIUI it’s mostly a legal distinction, based upon salvage rights.
