What is the origin of exclave?

Exclave “an outlying portion of a country entirely or mostly surrounded by the territory of another country” is modeled on the noun enclave, its opposite, using the Latin prefix ex- “out of, from.” Enclave comes from an older French term meaning “locked in,” from Latin clāvis “key,” which also gives rise to words such as clavicle, the collarbone, originally meaning “little key” in Latin, and conclave, a secret meeting, originally meaning “(room) with a key” in Latin. Exclave entered English in the late 1800s.