The bee's knees
Excellent - the highest quality.
It's difficult to know if we need an etymologist or an entomologist for this one.
Bees carry pollen back to the hive in sacs on their legs. It is tempting to explain this phrase as alluding to the concentrated goodness to be found around a bee's knee, but there's no evidence to support this explanation. It is also sometimes said to be a corruption of 'business', but there's no evidence to support that either.
Nor is there any connection with another earlier phrase, 'a bee's knee'. In the 18th century this was used as a synonym for smallness, but has since disappeared from the language, replaced more recently by the less polite 'gnat's bollock':
Mrs. Townley Ward - Letters, June 1797 in N. & Q. "It cannot be as big as a bee's knee."
'Bee's knees' began to be used in early 20th century America. Initially, it was just a nonsense expression that denoted something that didn't have any meaningful existence - the kind of thing that a naive apprentice would be sent to the stores to ask for, like a 'sky-hook' or 'striped paint'. That meaning is apparent in a spoof report in the New Zealand newspaper The West Coast Times in August 1906, which listed the cargo carried by the SS Zealandia as 'a quantity of post holes, 3 bags of treacle and 7 cases of bees' knees'. The teasing wasn't restricted to the southern hemisphere. The US author Zane Grey's 1909 story, The Shortstop, has a city slicker teasing a yokel by questioning him about make-believe farm products:
"How's yer ham trees? Wal, dog-gone me! Why, over in Indianer our ham trees is sproutin' powerful. An' how about the bee's knees? Got any bee's knees this Spring?"
This odd cartoon from the May 5th 1914 edition of the Fort Wayne Sentinel uses the term in exactly the same way:
[Text: Now dot I haf adopted Mr Skygack I suppose I haf to feed him. Vot does he eat? He likes bees' knees. Bees' knees? Yes, sure, he is very fond of them. Vell, I guess I got to catch some bees. Diss looks like a bee-hive.]
There's no profound reason to relate bees and knees other than the jaunty-sounding rhyme. In the 1920s it was fashionable to use nonsense terms to denote excellence - 'the snake's hips', 'the kipper's knickers', 'the cat's pyjamas/whiskers', 'the monkey's eyebrows' and so on. Of these, the bee's knees and the cat's whiskers are the only ones to have stood the test of time. More recently, we see the same thing - the 'dog's bollocks'.
The nonsense expression 'the bee's knees' was taken up by the socialites of Roaring 20s America and added to the list of 'excellent' phrases. A printed reference in that context appears in the Ohio newspaper The Newark Advocate, April 1922, in a piece on newly coined phrases entitles 'What Does It Mean?':
"That's what you wonder when you hear a flapper chatter in typical flapper language. 'Apple Knocker,' for instance. And 'Bees Knees.' That's flapper talk. This lingo will be explained in the woman's page under the head of Flapper Dictionary." [an 'apple knocker' is a rustic]
Clearly the phrase must have been new then for the paper to plan to take the trouble to define it. Disappointingly, they didn't follow up on their promise and 'the lingo' wasn't subsequently explained. Several U.S. newspapers did feature lists of phrases under 'Flapper Dictionary' headings. Although 'bee's knees' isn't featured, they do show the time as being a period of quirky linguistic coinage; for example, from one such Flapper Dictionary:
Kluck - dumb person.
Dumb kluck - worse than a kluck.
Pollywoppus - meaningless stuff.
Fly-paper - a guy who sticks around.
One tenuous connection between the bee's knees and an actual bee relates to Bee Jackson. Ms. Jackson was a dancer in 1920s New York and popularised the Charleston, being credited by some as introducing the dance to Broadway in 1924. She went on to become the World Champion Charleston dancer and was quite celebrated at the time.
It's not beyond the bounds of possibility that the expression became popular in reference to her and her very active knees, but 1924 post dates the origin of the phrase.
See other phrases that were coined in the USA.