exclave

You probably won't be surprised to learn that the key to "exclave" is found in "enclave." "Enclave" itself ultimately derives from the Latin word for "key," which is "clavis." It was adopted in the mid-19th century from Middle French enclaver ("to enclose"), which in turn derives (through Vulgar Latin) from in- and clavis. "Exclave" was formed about twenty years later by combining the prefix ex- and the "-clave" of "enclave." Other "clavis" descendants in English include "autoclave," "clavicle," "conclave," and "clavichord" ("an early keyboard instrument in use before the piano").