"act of shutting up in a cloister," 1863, as if from a noun of action formed in Latin from Latin claustrare, from claustrum (see cloister).
"morbid fear of being shut up in a confined space," coined 1879 (in article by Italian-born, French-naturalized Swiss-English physician Dr. Benjamin Ball), with -phobia "fear" + Latin claustrum "a bolt, a means of closing; a place shut in, confined place, frontier fortress" (in Medieval Latin "cloister"), from past participle of claudere "to close" (see close (v.)).
early 13c., cloystre, "a monastery or convent, a place of religious retirement or seclusion," from Old French cloistre "monastery, convent; enclosure" (12c., Modern French cloître), from Medieval Latin claustrum "portion of monastery closed off to laity," from Latin claustrum (usually in plural, claustra) "place shut in, enclosure; bar, bolt, means of shutting in," from past participle stem of claudere "to close, shut" (see close (v.)).
"The original purpose of cloisters was to afford a place in which the monks could take exercise and recreation" [Century Dictionary]. Spelling in French influenced by cloison "partition." Old English had clustor, clauster in the sense "prison, lock, barrier," directly from Latin, and compare, from the same source, Dutch klooster, German Kloster, Polish klasztor.
From c. 1300 in English as "covered walk running round the walls of a monastic building or large church;" from late 14c. in the general sense "colonnade round an open court."
"pulpit or platform from which a speaker addresses an audience," 1540s, originally in an ancient Roman context, from Latin rostrum, the name of the platform stand for public speakers in the Forum in ancient Rome. It was decorated with the beaks of ships taken in the first naval victory of the Roman republic, over Antium, in 338 B.C.E., and the Latin word's older sense is "end of a ship's prow," literally "beak, muzzle, snout," originally "means of gnawing," an instrument noun from rodere "to gnaw" (see rodent).
The beaks were an ancient form of ram, a beam spiked with pointed iron for the purpose of sinking other vessels. For the form, compare claustrum "lock, bar," from claudere "to shut." The extended sense, in reference to any platform for public speaking, is attested by 1766.
The classical plural is rostra, though in English this is more common in the original "ship's beak" sense and -rums often is used in the secondary sense.