Where does “math” come from?

math (Middle English) comes from Old English mǣþ, from Proto-Germanic mēþą, from Proto-Indo-European h₂meh₁- — to mow, reap.

Ancestry of “math”, step by step

Step	Language	Word	Meaning
1	Old English	mǣþ	measure, degree, proportion; measure, degree; a...
2	Proto-Germanic	mēþą	a mowing
3	Proto-Indo-European	h₂meh₁-	to mow, reap

math — Etymology, Origin & Word History | Etymology Explorer