Wiktionary
keen as mustard
a. (context simile English) To be very keen, enthusiastic, eager.
Usage examples of "keen as mustard".
These new guys got out of the city just ahead of Finn's people, and they're keen as mustard to get some payback, by helping us with our site set-up.
He was keen as mustard, though, and slaughtered stags like a Ghazi on hashish - you'll hardly credit it, but his notion of sport was that a huge long trench should be dug so that we could sneak up on the deer unobserved.
We feed him once a day, and not too much then, so that he is always as keen as mustard.