Green’s Dictionary of Slang

bee’s knees n.

also bullock’s bollocks, cuckoo’s chin, duck’s nuts, elephant’s (fallen) arches, elephant’s manicure, gnat’s elbow, nit’s tits, owl’s bowels, snail’s ankles, snake’s hips, ...toenails, turkey’s elbow, turtle’s neck, worm’s ribs
[nuts n.2 (1)]

1. a superior person, or someone who poses as such.

2. of things, the best, the superlative.