With later shortening in the vowel. The this means "a divide of land" (sense 2), attested before in watershed (18th century) seems to be an outgrowth of previously physical senses of the phrase, as "the Section of one's hair" or "a passageway in between the threads of a warp on https://sheds20864.hyperionwiki.com/704504/duramax_for_dummies