What does Boxing Day have to do with boxing?
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.
narrow strip of land connecting two large land areas otherwise separated by the sea. Unquestionably the two most famous are the Isthmus of Panama, connecting North and South America, and the Isthmus of Suez, connecting Africa and Asia. Historically the Isthmus of Corinth was of major importance because it connected what otherwise would be the island of the Peloponnese with the rest of the Greek peninsula. Isthmuses are of great importance in plant and animal geography because they offer a path for the migration of land plants and animals between the two land masses they connect.