English Idioms and Phrases: The bee's knees
If something is 'the bee's knees' it is the best or very very good.
"That suit is amazing, you look the bee's knees".
This phrase has a very interesting origin as it's meaning suddenly changed. You can see the current meaning above, but when the phrase came into use in the 18th century it meant something unimportant or inconsequential.
Later in the 1920's it's meaning was changed to it's current one. In this time it became fashionable to use phrases that had no meaning to describe something as being excellent or being the best. There were many phrases used at the time in the form of a nonsensical compound noun (that had a pattern of being an animal followed by a body part) - for example 'the monkey's eyebrows", "the snake's hips" (which are both no longer used in the English language) and "the cat's whiskers" (which is still in use). All of these phrases mean 'the best' and it seems that the phrase 'the bee's knees', which follows the pattern of the other phrases, was drafted in and it's original meaning was replaced with the new one to match the other similar phrases.
In recent history we have seen a new phrase in the form of "the dog's bollocks" (bollocks is a rude word for testicles) which also has a similar meaning.
Alternatives or Synonyms
the best, very good.
The cat's whiskers, the dog's bollocks (swear word)