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To show no interest or concern for an activity.

The opposite of the phrase 'keen as mustard'.

The phrase is believed to come from the fact that mayonaise is the opposite to mustard. They are both condiments but mayo is to an egg what mustard is to a seed.
Person 1: Have you written much of your essay?
Person 2: Nope, I'm about as keen as mayo, all I've done is plagiarise 2,000 words.
by DennisCarbon August 25, 2011
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