With later shortening of the vowel. The meaning "a divide of land" (sense 2), attested before in watershed (18th century) seems to generally be an outgrowth of earlier physical senses from the word, as "the A part of a person's hair" or "a passageway involving the threads of a warp https://franciscotagmr.thebindingwiki.com/7476658/4x10_fundamentals_explained