c. 1200, "wasteland, wilderness, barren area," wooded or not, from Old French desert (12c.) "desert, wilderness, wasteland; destruction, ruin" and directly from Late Latin desertum (source of Italian diserto, Old Provençal dezert, Spanish desierto), literally "thing abandoned" (used in Vulgate to translate "wilderness"), noun use of neuter past participle of Latin deserere "forsake" (see desert (v.)).

Sense of "waterless, treeless region of considerable extent" was in Middle English and gradually became the main meaning. Classical Latin indicated this idea with deserta, plural of desertus. Commonly spelled desart in 18c., which is unetymological, but it avoids confusion with the two other senses of the word. 

Every important worker will report what life there is in him. It makes no odds into what seeming deserts the poet is born. Though all his neighbors pronounce it a Sahara, it will be a paradise to him; for the desert which we see is the result of the barrenness of our experience. [Thoreau, Journal, May 6, 1854]

city in Minnesota, U.S., founded 1850s and named for French pioneer explorer Daniel Greysolon, sieur du Luth, "the Robin Hood of Canada," the leader of the coureurs de bois, who passed through the region in 1678 on a mission into the wilderness.

1757, "military position detached from the main body of troops or outside the limits of a camp," from out- + post (n.2). Originally in George Washington's letters. Phrase outpost of Empire (by 1895) "remotest territory of an empire" in later use often echoes Kipling:

There he shall blaze a nation’s ways with hatchet and with brand,

Till on his last-won wilderness an Empire’s outposts stand!

1530s, "pertaining to the earliest period, of a primitive style, ancient," from French pristin and directly from Latin pristinus "former, early, original," from Old Latin pri "before," from PIE root *per- (1) "forward," hence "in front of, before, first." Meaning "unspoiled, untouched, pure" is from 1899 (implied in a use of pristinely) is extended from such expressions as pristine wilderness, but according to OED [2nd ed. print], this is regarded as ignorant "by many educated speakers."

Old English borrowing from Late Latin manna, from Greek manna, from Hebrew mān, probably literally "substance exuded by the tamarisk tree," but used in Greek and Latin specifically with reference to the substance miraculously supplied to the Children of Israel during their wandering in the Wilderness (Exodus xvi.15). The Hebrew word often is referred to man "a gift." Meaning "spiritual nourishment" is attested from late 14c. Generalized sense of "something provided unexpectedly" is from 1590s.