The closable shell of box turtles is of obvious survival value in providing protection for the soft parts of the body.
c.1200, "to shut, cover in," from Old French clos- (past participle stem of clore "to shut, to cut off from"), 12c., from Latin clausus, past participle of claudere "to shut, close; to block up, make inaccessible; put an end to; shut in, enclose, confine" (always -clusus, -cludere in compounds).
The Latin word might be from the possible PIE root *klau- "hook, peg, crooked or forked branch" (used as a bar or bolt in primitive structures); cf. Latin clavis "key," clavus "nail," claustrum "bar, bolt, barrier," claustra "dam, wall, barricade, stronghold;" Greek kleidos (genitive) "bar, bolt, key," klobos "cage;" Old Irish clo "nail," Middle Irish clithar "hedge, fence;" Old Church Slavonic ključi "hook, key," ključiti "shut;" Lithuanian kliuti "to catch, be caught on," kliaudziu "check, hinder," kliuvu "clasp, hang;" Old High German sliozan "shut," German schließen "to shut," Schlüssel "key."
Also partly from Old English beclysan "close in, shut up." Intransitive sense "become shut" is from late 14c. Meaning "draw near to" is from 1520s. Intransitive meaning "draw together, come together" is from 1550s, hence the idea in military verbal phrase close ranks (mid-17c.), later with figurative extensions. Meaning "bring to an end, finish" is from c.1400; intransitive sense "come to an end" is from 1826. Of stock prices, from 1860. Meaning "bring together the parts of" (a book, etc.) is from 1560s. Related: Closed; closing.
late 14c., "strictly confined," also "secret," from Old French clos "confined; concealed, secret; taciturn" (12c.), from Latin clausus "close, reserved," past participle adjective from claudere "stop up, fasten, shut" (see close (v.)); main sense shifting to "near" (late 15c.) by way of "closing the gap between two things." Related: Closely.
Meaning "narrowly confined, pent up" is late 14c. Meaning "near" in a figurative sense, of persons, from 1560s. Meaning "full of attention to detail" is from 1660s. Of contests, from 1855. Close call is from 1866, in a quotation in an anecdote from 1863, possibly a term from the American Civil War; close shave in the figurative sense is 1820, American English. Close range is from 1814. Close-minded is attested from 1818. Close-fisted "penurious, miserly" is from c.1600.
late 14c., "act of closing, conclusion, termination," from close (v.). Also in early use "enclosure, enclosed space" (late 13c.), from Old French clos, noun use of past participle.
"tightly, with no opening or space between," from close (adj.).