Can't cut the mustard.

Whatever the origins of 'can't cut the mustard', they are about as clear as mustard, the expression 'too old to cut the mustard' is always applied to to men today and conveys the idea of sexual inability. ' Can't cut the mustard', however, means not to be able to handle any job for any reason, not just because of old age. Preceeding the derivation of 'too old to cut the mustard' by about half a century, it derives from the expression 'to be the mustard'. "Mustard" was slang for the " genuine article" or " main attraction" at the time. Perhaps someone cutting up to show that he was 'the mustard', or the greatest, was said 'to cut the mustard' and the phrase was later meant to mean to be able to fill the bill or or do the important or main job. In any case, O. Henry first used the words in this sense in his story "Heart of the West" (1907) when he wrote: " I looked around and found a proposition that exactly cut the mustard". Today, 'can't cut the mustard' is usually 'can't cut it' or 'can't hack it'. A recent variant on 'too old to cut the mustard' is 'if you can't cut the mustard, you can lick the jar'.
QPB Encyclopedia of Word and
Phrase Origins
By Robert Hendrickson