Originated c. 1925 in the US but the precise etymology is not known for certain. There are several suggested origins but it may simply have been in imitation of the numerous animal related nonsense phrases popular in the 1920s such as the cat's pyjamas, cat's whiskers, cat's meow, gnat's elbow, monkey's eyebrows etc. A popular folk etymology has the phrase referring to World Champion Charleston dancer Bee Jackson. Another suggestion is that the phrase is a corruption of business but this may be a back-formation. The singular bee's knee is attested from the late 18th century meaning something small or insignificant in the phrase big as a bee's knee. Also as weak as a bee's knee is attested in Ireland (1870). It is possible that the bee's knees is a deliberate inversion of this meaning, but is not attested.