Latin frontus had a sense of "ability to blush," but the literal sense of effrontery often has been taken to be "putting forth the forehead." Forehead in Johnson's Dictionary (1755) has a secondary sense of "impudence; confidence; assurance; audaciousness; audacity." English had an earlier verb effront "treat with effrontery" (17c.).
1540s, "confident, intrepid, daring," from French audacieux, from audace "boldness," from Latin audacia "daring, boldness, courage," from audax "brave, bold, daring," but more often "bold" in a bad sense, "rash, foolhardy," from audere "to dare, be bold." In English, the bad sense of "shameless, unrestrained by propriety" is attested from 1590s. Related: Audaciously; audaciousness.
late 14c., "impudent, shameless, presumptuous," from Old French mal apert "over-ready, impudent," literally "ill-skilled," from mal "badly" (see mal-) + apert "skillful," variant of espert "experienced, skillful, clever" (from Latin expertus; see expert (adj.)). Attested from c. 1300 as the name of the personification of impudence. From mid-15c. as an adverb, "impudently, presumptuously." Related: Malapertly; malapertness.
Sense "foremost part of anything" emerged in the English word mid-14c.; sense of "the face as expressive of temper or character" is from late 14c. (hence frontless "shameless," c. 1600). The military sense of "foremost part of an army" (mid-14c.) led to the meaning "field of operations in contact with the enemy" (1660s); home front is from 1919. Meaning "organized body of political forces" is from 1926. Sense of "public facade" is from 1891; that of "something serving as a cover for illegal activities" is from 1905. Adverbial phrase in front is from 1610s. Meteorological sense first recorded 1921.
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" (source also of Old Norse opinn, Swedish öppen, Danish aaben, Old Saxon opan, Old Frisian epen, Old High German offan, German offen "open"), from PIE root *upo "under," also "up from under," hence also "over." 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" (such as 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-door in reference to international trading policies is attested from 1856. Open season is recorded by 1895 of game; figuratively (of persons) by 1914. 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-faced, of sandwiches, etc., "without an upper layer of bread, etc.," by 1934. Open marriage, one in which the partners sleep with whomever they please, is by 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.