What's the meaning of the phrase 'Pass muster'?
To 'pass muster' originally meant 'pass military inspection without censure'. More recently it is used more generally to mean 'come up to the required standard'.
What's the origin of the phrase 'Pass muster'?
This little expression was hardly idiomatic when it was first coined - the words that make it up were used literally. As early as the 14th century muster was used as the name of the assembling together of people or things. John Wycliffe used it in his Bible, circa 1382.
In the 15th century muster began to be used specifically to refer to the act of calling together soldiers, sailors, prisoners for inspection, exercises or counting and that meaning is still in common use.
To pass muster was simply to meet the standards of the inspection.
In more recent times, if we count the 16th century as recent that is, the expression has been used figuratively. Passing muster then became 'meet the required standard' in circumstances where no soldiers or other groups were gathered together.
An early example of 'pass muster' in print with the generalised meaning is from the English poet George Gascoigne, in The Adventures of Master F.J., 1573:
It was devised in great disquiet of mind, and written in rage, yet have I seene much worse passe the musters.
At this point it's worth mentioning that this expression has no connection with another phrase with a similar sound and meaning - 'cut the mustard'. In fact it has nothing to do with mustard at all.
The often seen spelling of 'pass mustard' is just a mistake. Misspellings of this sort, caused by mishearings, for example 'Old-timer's Disease' for Alzheimer Disease, are called eggcorns.