bee's knees (n.)
1923, a survivor of a fad around this year for slang terms denoting "excellence" and based on animal anatomy. Also existed in the more ribald form bee's nuts. Other versions that lasted through the century are cat's whiskers (1923), cat's pajamas, cat's meow. More obscure examples are canary's tusks, cat's nuts and flea's eyebrows. The fad still had a heartbeat in Britain at the end of the century, as attested by the appearance of dog's bollocks in 1989. Bee's knee was used as far back as 1797 for "something insignificant."
updated on October 12, 2012