Why is the ninth month called September?
Old English open "not closed down, raised up" (of gates, eyelids, etc.), also "exposed, evident, well-known, public," often in a bad sense, "notorious, shameless;" from Proto-Germanic *upana, literally "put or set up" (cf. Old Norse opinn, Swedish öppen, Danish aaben, Old Saxon opan, Old Frisian epen, Old High German offan, German offen "open"), from PIE *upo "up from under, over" (cf. Latin sub, Greek hypo; see sub-). Related to up, and throughout Germanic the word has the appearance of a past participle of *up (v.), but no such verb has been found. The source of words for "open" in many Indo-European languages seems to be an opposite of the word for "closed, shut" (e.g. Gothic uslukan).
Of physical spaces, "unobstructed, unencumbered," c.1200; of rooms with unclosed entrances, c.1300; of wounds, late 14c. Transferred sense of "frank, candid" is attested from early 14c. Of shops, etc., "available for business," it dates from 1824. Open-handed "liberal, generous" is from c.1600. Open door in reference to international trading policies is attested from 1856. Open season is first recorded 1896, of game; and figuratively 1914 of persons. Open book in the figurative sense of "person easy to understand" is from 1853. Open house "hospitality for all visitors" is first recorded 1824. Open-and-shut "simple, straightforward" first recorded 1841 in New Orleans. Open marriage, one in which the partners sleep with whomever they please, is from 1972. Open road (1817, American English) originally meant a public one; romanticized sense of "traveling as an expression of personal freedom" first recorded 1856, in Whitman.
early 13c., "an aperture or opening," from open (adj.). Meaning "public knowledge" (especially in out in the open) is from 1942, but cf. Middle English in open (late 14c.) "manifestly, publicly." The sense of "an open competition" is from 1926, originally in a golf context.
Old English openian "to open, open up, disclose, reveal," also intransitive, "become manifest, be open to or exposed to," from Proto-Germanic *opanojan (cf. Old Saxon opanon, Old Norse opna "to open," Middle Dutch, Dutch openen, Old High German offanon, German öffnen), from the source of open (adj.), but etymology suggests the adjective is older. Open up "cease to be secretive" is from 1921. Related: Opened; opening.