1341: math and aftermath Aug 13, 2018
The first entry for 'math' in many dictionaries begins with the definition as a verb meaning 'to mow' rather than 'mathematics'. By frequency 'math' as 'mathematics' is used more often, but there are more words derived from the meaning as 'to mow', so it is number one. Included in these would be 'day's math', 'undermath' and 'lattermath', all of which are nouns, and all are fairly rare. The only one that is more common is 'aftermath', which is not related to 'mathematics' at all, and refers to a field after it has been mowed.
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