1550s, from Latin isthmus, from Greek isthmos "narrow passage, narrow neck of land," especially that of Corinth, of unknown origin, perhaps from eimi "to go" + suffix -thmo (cf. ithma "a step, movement").
isthmus isth·mus (ĭs'məs)
n. pl. isth·mus·es or isth·mi (-mī')
A constriction or narrow passage connecting two larger parts of an organ or other anatomical structure.
A narrow strip of land that connects two larger bodies of land and has water on both sides.