As keen as mustard
There are many similes in English that have the form 'as x as y' (see this list). 'As keen as mustard' is typical and, although rather archaic, is worth a closer look - if only to dispel a myth about its origin.
The Olde England of folk-memory conjures up pictures of ale-quaffing yeomen tucking into sides of roast beef. That may be fanciful, but the long-standing enthusiasm for the Sunday roast was real, as reflected in the words of Richard Leveridge's 1735 song Roast beef of old England:
When mighty Roast Beef
Was the Englishman's food,
It ennobled our brains
And enriched our blood...
Mustard was an essential accompaniment to beef. It became associated with vigour and enthusiasm because it added zest and flavour. By the early 20th century, the association was so strong that the word was used like this:
1925 E. Wallace, in King by Night: "That fellow is mustard."
People and things weren't just like mustard, they were mustard. The phrase 'hot stuff' comes from the same notion.
Mustard's hot and zesty reputation wasn't limited to food. It was also considered a cure for colds and fevers. Here's part of the instructions for the use of a mustard plaster poultice, which was a 19th century remedy for 'chest congestion', used then by The Church of Jesus Christ of Latter-day Saints.
"As soon as the chest turns quite pink, remove the poultice."
Don't try this at home, or if you do, you might like to remove the poultice a little earlier. The church is now careful to say "These remedies were used by pioneers and are not recommended for today's use."
The phrase is first recorded in William Walker's exhaustively titled Phraseologia Anglo-Latina, or phrases of the English and Latin tongue; together with Paroemiologia Anglo-Latina, 1672:
"As keen as mustard."
The context there is missing but the meaning is clear in F. Smith's Clod-pate's Ghost, 1679:
"You shall see a man as hot as Mustard against Plot and Plotters."
These are slightly pre-dated by a similar phrase 'the keenest mustard', from 1658.
So, back to the origin. A company called Keen and Sons was one of the earliest manufacturers of the condiment. They were taken over by Colman's in 1903, although the brand name persists and mustard is still available under that name.
'As keen as mustard' and a well-known company called Keen that made mustard. Surely we have a winner?
Unfortunately not. The phrase 'as keen as mustard' is known from 1672, the century before the company was formed in 1742. The caption on the accompanying image is probably too small to read online, but says [Keen's Mustard] "First manufactured in the reign of George 2nd AD 1742."
See other 'as x as y similes'.