Keen as mustard
As keen as mustard means to be very enthusiastic and dates from the late 17th century. Since the 1400s, the word keen has enjoyed at least three meanings: sharp as in a keen knife blade, sharp or pungent as in taste, and finally keen as in eager and enthusiastic. Clearly, the latter two meanings are applicable here. As for mustard, there has long been an association with the zest and piquancy of mustard with people’s behaviour or demeanour, as in keen as mustard which is first cited from 1672. See also Cut the mustard.