Where does “Middlesex” come from?

Middlesex derives from Old English Seaxe, Old English Seaxa, and Proto-Germanic midlą meaning "middle," traced to Proto-Indo-European médʰyos with the same sense.

Ancestry of “Middlesex”, step by step

Step	Language	Word	Meaning
1	Old English	Middelseaxan	—
2	Old English	middel	middle
3	Proto-West Germanic	*midl	—
4	Proto-Germanic	midlą	middle
5	Proto-Germanic	midjō	middle, midst
6	Proto-Indo-European	médʰyos	middle
7	Proto-Indo-European	medʰi-	with, among; with, along
8	Proto-Indo-European	me	in the middle of, near, by, around, with

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Middlesex — Etymology, Origin & Word History | Etymology Explorer