Etymology
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back off (v.)
"retreat, stop annoying someone," by 1938, from the verbal phrase, from back (v.) + off (adv.).
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get back (v.)
c. 1600 (intransitive) "to return;" 1808 (transitive) "to recover (something);" from get (v.) + back.(adv.). Meaning "retaliate" is attested by 1888.
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back up (v.)
1767, "stand behind and support," from back (v.) + up (adv.). Meaning "move or force backward" is by 1834. Of water prevented from flowing, by 1837.
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back down (v.)
in figurative sense of "withdraw a charge," 1859, American English, from notion of descending a ladder, etc. (the literal sense by 1849); from back (v.) + down (adv.).
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back seat (n.)

also back-seat, 1832, originally of coaches, from back (adj.) + seat (n.). Used figuratively for "less or least prominent position" by 1868. Back-seat driver attested by 1923.

You know him. The one who sits on the back seat and tells the driver what to do. He issues a lot of instructions, gives advice, offers no end of criticism and doesn't do a bit of work. ["The Back Seat Driver," Wisconsin Congregational Church Life, May 1923]
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silver bullet (n.)
"very effective, almost magical remedy," 1808. The belief in the magical power of silver weapons to conquer foes goes back at least to ancient Greece (as in Delphic Oracle's advice to Philip of Macedon).
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au revoir (interj.)
1690s, French, "good-bye for now," literally "to the seeing again." From au "to the" (see au) + revoir "see again, see in turn" (Old French reveoir, 12c.), from Latin revidere, from re- "back, again" (see re-) + videre "to see" (see vision).
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lingua franca (n.)
1620s, from Italian, literally "Frankish tongue." A stripped-down Italian peppered with Spanish, French, Greek, Arabic, and Turkish words, it began as a form of communication in the Levant. The name probably is from the Arabic custom, dating back to the Crusades, of calling all Europeans Franks (see Frank). Sometimes in 17c. English sources also known as Bastard Spanish.
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contra-dance (n.)

"dance by four couples placed opposite to each other and making the same steps and figures," 1803, from French contre-danse, altered from English country dance by folk etymology from French contra "against," suggested by the arrangement of the partners in the dance. The dances and the name were taken up in France c. 1720s and from there passed to Spain and Italy (Spanish, Italian contra danza) then back to English.

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absolute zero (n.)
"lowest possible temperature which the nature of heat admits" (determined to be –273 centigrade, –458 Fahrenheit), the idea dates back to 1702 and its general value was guessed to within a few degrees soon thereafter, but not precisely discovered until Lord Kelvin's work in 1848. It was known by many names, such as infinite cold, absolute cold, natural zero of temperature; the term absolute zero was among them by 1806.
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