bee
I came across the expression “be-all and end-all” in a document, which I chose to translate in French as panacée. I knew it took its origins in a Shakespeare play, but couldn’t remember which one, so I went in search of an answer. It was Macbeth:

Macbeth says: If it were done when ’tis done, then ’twere well it were done quickly: if the assassination could trammel up the consequence, and catch, with his surcease, success; that but this blow might be the be-all and end-all here.

What I read next was more surprising:

After years of use, ‘the be-all and [the] end-all’ became shortened to: the Bs and Es. […]
As this was said, over time (if you repeat this fast, you will see), it sounds like ‘the bee’s knees’.
Future Perfect

Really? That’s not what I found when I researched the expression “the bee’s knees” a few years ago. I turned to a more authoritative source and this is what I found:

It’s sometimes explained as being from an Italian-American way of saying business or that it’s properly Bs and Es, an abbreviation for be-alls and end-alls. Both are without doubt wrong. Bee’s knees is actually one of a set of nonsense catchphrases from 1920s America, the period of the flappers, speakeasies, feather boas and the Charleston.
World Wide Words

Some words just intrigue so much that people love working out funny, but ultimately wrong, origins. A friend confidently told me this weekend that “loo” came from regardez l’eau, which servants cried before tipping chamber-pots outside the window to warn passers-by. I voiced my scepticism and promised I would look into it. As I suspected, there are many possible explanations for the origin of “loo”:

There are several theories about the origin of this common term for a familiar article of sanitary furniture. The first, and most popular, is that it is derived from the cry of “gardyloo” (from the French regardez l’eau or “watch out for the water”) which was shouted by medieval servants as they emptied the chamber-pots out of the upstairs windows into the street. This is historically problematic, since by the time the term “loo” is recorded, the expression “gardyloo” was long obsolete. A second theory is that the word derives from a polite use of the French term le lieu (the place) as a euphemism. Unfortunately, documentary evidence to support this idea is lacking. A third theory, favoured by many, refers to the trade name “Waterloo”, which appeared prominently displayed on the iron cisterns in many British outhouses during the early 20th century. This is more credible in terms of dates, but corroborating evidence is still frustratingly hard to find. Various other picturesque theories also circulate, involving references to doors numbered “00” or people called “Looe”.
Ask Oxford

Conclusion: folk etymology is fun, but if you are interested in the real origin of words, you have to be prepared to do a lot of research and to often be disappointed.
Bee photo by anna316