c.1300, from O.Fr. clostre
or O.E. clauster,
both from M.L. claustrum
"portion of monastery closed off to laity," from L. claustrum
"place shut in, bar, bolt, enclosure," from pp. stem of claudere
(v.)). Sense of "enclosed space" extended to "place of religious seclusion." The verb is recorded from 1581.