The Bee's Knees

When you refer to something as 'the bee's knees', it means that it is of excellent or very high quality.

The origin of this expression is largely unknown, although there are a number of theories. Some people believe that it is a reference to the fact that bees carry pollen in sacks on their knees, and that the expression therefore alludes to this concentrated goodness. Others maintain that the saying is just a corruption of the word 'business'.

Another suggestion is that the 'bee' in question was actually Bee Jackson, a world champion Charleston dancer who was very popular in New York in the 1920's!

https://www.ecenglish.com/learnenglish/lessons/idiom-day-the-bees-knees