As keen as mustard

You may know that keen means enthusiastic but why as keen as mustard?

Mustard, and particularly Coleman's English mustard, is very spicy and in this sense keen is a synonym of hot.

It could also mean extreme in some way.

There was a bit of a keen wind blowing through the valley.

So keen as mustard is a play on the double meaning of keen but for this idiom it means very enthusiastic.

Here's a classic, British advert with some aristocrats and a mustard crisis!