Origin of the county’s name.

Any history buffs on here able to explain the origins of “Westchester” as a name for the county?

Westchester implies the existence of a central “Chester”. We have eastchester and port Chester as towns, but what about a town called Westchester?

This post may or may not be inspired by Phoebe on FRIENDS saying that Westchester was “the worst of the Chester’s” in one of the later seasons.