With afterwards shortening in the vowel. The meaning "a divide of land" (perception 2), attested previously in watershed (18th century) seems to be an outgrowth of previously physical senses with the word, as "the Component of 1's hair" or "a passageway involving the threads of a warp over a loom," https://detection36790.wiki-cms.com/6853882/the_smart_trick_of_4x10_that_no_one_is_discussing