Postholes served as makeshift banks

What is a “posthole bank?” I’ve seen the term in connection with stories of coin finds with a metal detector.
The nickname was applied to the early day practice of homesteaders who were miles from any bank and who used the opportunity of a posthole dug near their house to bury coins or other valuables for safekeeping.

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Is “rap” a slang term or a legitimate coin denomination?
My old Webster describes it as a slang term for an early 18th century counterfeit half penny in Ireland and goes on to cite it as the source for the saying, “I don’t give a rap.”

An obscure coin nickname for you to dig up a meaning for us: a “Robertson?”
Frey’s Numismatic Dictionary missed this one, but the answer is that it was applied to clipped, mutilated or fake gold coins of Spain, Portugal and England. An infamous General Robertson was the English Governor of New York during the British occupation of that city and feathered his nest by demanding genuine coins in payment, in turn paying out clipped coins for supplies. The nickname was applied first to the clipped pieces and spread later to the other frauds.

What is a “trap” coin?
At first I thought this might be some sort of variation of the screw talers that were hollowed out to serve as lockets, but “trap” is a slang term applied to coins that have hidden damage, something that doesn’t show up except under strong magnification or rim damage that is hidden by the holder or slab. It also seems to have been applied to coins that are overpriced.

Should I use aluminum foil to store my coins?
Despite some of the old books – and one recent one – aluminum foil is definitely dangerous for a coin and absolutely should not be used for storing them. I’ve seen some beautiful coins ruined by contact with the foil.

An old book on coins mentions cleaning them with argol. What is it, and where can I get it?
Argol is a pinkish crust of crude tartar that forms in wine barrels and was used in several of the world mints to polish coin planchets that were tumbled in a revolving barrel. Each mint has its own secret formula for cleaning planchets, but I doubt if argol would work very well on a coin.