c. 1400, frowntere, "front line of an army;" early 15c., fronture, "borderland, part of a country which faces another," from Old French frontiere "boundary-line of a country," also "frontier fortress; front rank of an army" (13c.), noun use of adjective frontier "facing, neighboring," from front "brow" (see front (n.)). In reference to North America, "part of the country which is at the edge of its settled regions" from 1670s. Later it was given a specific sense:
What is the frontier? ... In the census reports it is treated as the margin of that settlement which has a density of two or more to the square mile. [F.J. Turner, "The Frontier in American History," 1920]
also marquess, c. 1300, marchis, title of nobility, from Old French marchis, marcheis, marquis, etymologically "a prefect of the marches, ruler of a border area," from Old French marche "frontier," from Medieval Latin marca "frontier, frontier territory" (see march (n.1)). Originally the ruler of border territories in various European regions (compare Italian marchese, Spanish marqués, and see margrave); later a mere title of rank, below duke and above earl or count. Related: Marquisate.
by 1670s, from Russian or Polish Ukraina, a specific use of ukraina "border, frontier," according to Room, from Old Russian oukraina, from ou "by, at" + kraj region. He also notes that "The territory was so called because it was the borderland or 'frontier zone' of medieval Russia at the time of the Tatar invasion in the 13th century."
Ukraine was formerly also known as Little Russia, so called by contrast with Great Russia, when the medieval principality here became separated from 'mainstream' czarist Russia as a result of the Mongol invasion. [Room, 2006]
"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.)).