Why does "all to the mustard" mean excellent?

Question

While reading P.G. Wodehouse's The Inimitable Jeeves I came across a fascinating expression of "all to the mustard!" It is defined as meaning excellent. Why? Can anyone please help me understand this expression?

Answer 1

Kris' link offers this explanation:

[During] the beginning of the century, “to be the proper mustard” was a slang phrase meaning to be the genuine article, possibly because some so-called “mustard” of that period would not pass today’s pure food requirements.

From that, immediately, came “all to the mustard,” that is, all one could ask for, fine and dandy, “copesetic,” as the late Bill Robinson would say.