adumbrate - In Latin, umbra means "shadow," the sort of thing that an umbrella makes. To "adumbrate" is to bring something forth from those shadows - to explain a mystery, for instance. Strangely, though, "adumbrate" can also mean to cast a shadow over something, which makes the word an autoantonym, a word that contains contradictory definitions.
Jeep - There are two theories for the name of the ever-popular vehicle introduced in 1940 for military use. The first, and probably correct, owes to the acronym given to it by the Army: GP, meaning "general purpose." The second, which is more attractive but a touch far-fetched, is the vehicle was named for a character in a 19302-era cartoon whose cry was "jeep, jeep" and who was able to clamber over any obstacle.
funny bone - The spot where the arm bones called the ulna and the humerus join is particularly sensitive, for there the skin is about the thinnest it is anywhere on the body and the nerves rise close to the surface. Banging the joint against a table or counter is anything but funny, but some wag in the early 1800s decided that "humerus" and "numerous" made for a nice pun - thus, "funny bone."
stool pigeon - A "stool pigeon" was an unfortunate bird tied to a stand to lure hunting birds, such as hawks and falcons, into a trap by which they could be caught and then trained. In English slang, a "stool pigeon was a minor criminal whom the police, promising a lighter sentence, would send out to entrap a bigger crook. That sense has largely disappeared, and now we use it to mean a prisoner who informs on his or her fellow inmates.
hypothesis - A builder in ancient Greece, setting about making on of that land's magnificent structures, would start with a "hypothesis": that is, a foundation, from the words meaning "to place beneath." In scientific parlance, a "hypothesis" is the foundation of a theory, which may or may not be proven wrong but at least has some basis.