keen as mustard
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keen as mustardmainly BRITISH, OLD-FASHIONED
If someone is keen as mustard they are very eager to do something. I have an adult pupil who scored very low in assessments but is keen as mustard. Note: You can also describe someone as mustard-keen. Sir Richard was mustard-keen to say his bit. Note: `Keen' means enthusiastic, but is also used to mean sharp when referring to the blade or cutting edge of a tool or weapon. An acidic or sour taste can also be referred to as sharp, so enthusiasm is being likened to the sharp taste or `edge' given to food by mustard.
keen as mustardextremely eager or enthusiastic. British informal
Keen is used here to mean ‘operating on the senses like a sharp instrument’.