Online Etymology Dictionary

authentic (adj.): mid-14c., "authoritative," from Old French autentique (13c., Modern French authentique) "authentic; canonical," and directly from Medieval Latin authenticus, from Greek authentikos "original, genuine, principal," from authentes "one acting on one's own authority," from autos "self" (see auto-) + hentes "doer, being," from PIE *sene- "to accomplish, achieve." Sense of "entitled to acceptance as factual" is first recorded mid-14c.

Traditionally in modern use, authentic implies that the contents of the thing in question correspond to the facts and are not fictitious; genuine implies that the reputed author is the real one; but this is not always maintained: "The distinction which the 18th c. apologists attempted to establish between genuine and authentic ... does not agree well with the etymology of the latter word, and is not now recognized" [OED].
authenticate (v.): "verify, establish the credibility of," 1650s, from Medieval Latin authenticatus, past participle of authenticare, from authenticus (see authentic). Related: Authenticated; authenticating.
authenticity (n.): 1760; see authentic + -ity. Earlier form was authentity (1650s).
effendi (n.): Turkish title of respect, equivalent to English sir, 1610s, from Turkish efendi, title of respect applied to professionals and officials, corruption of Greek authentes "lord, master" (in Modern Greek aphentes; see authentic).
inauthentic (adj.): 1783, from in- (1) "not, opposite of" + authentic. Related: Inauthentically.
authentication (n.): 1788, noun of action from authenticate (v.).
elderly (adj.): "bordering on old age, somewhat old," 1610s, from elder + -ly (1). Now, generally, "old." Old English ealdorlic meant "chief, princely, excellent, authentic." Old English also had related eldernliche "of old time," literally "forefatherly."
dandelion (n.): early 15c., earlier dent-de-lioun (late 14c.), from Middle French dent de lion, literally "lion's tooth" (from its toothed leaves), translation of Medieval Latin dens leonis. Other folk names, like tell-time refer to the custom of telling the time by blowing the white seed (the number of puffs required to blow them all off supposedly being the number of the hour), or to the plant's more authentic diuretic qualities, preserved in Middle English piss-a-bed and French pissenlit.
cuckold (n.): mid-13c., kukewald, from Old French cucuault, from cocu (see cuckoo) + pejorative suffix -ault, of Germanic origin. So called from the female bird's alleged habit of changing mates, or her authentic habit of leaving eggs in another bird's nest.

In Modern French the identity is more obvious: Coucou for the bird and cocu for the betrayed husband. German Hahnrei (13c.), from Low German, is of obscure origin. The second element seems to be connected to words for "ardent," and suggests perhaps "sexually aggressive hen," with transferal to humans, but Kluge suggests rather a connection to words for "capon" and "castrated." Related: Cuckoldry.
historic (adj.): 1660s, "of or belonging to history," probably a back-formation from historical, perhaps influenced by French historique. Meaning "what is noted or celebrated in history" is from 1794.

Though both historic and historical have been used in both senses by respected authors, now the tendency is to reserve historic for what is noted or celebrated in history; historical for what deals with history. The earliest adjective form of the word in English was historial (late 14c., from Late Latin historialis), which meant "belonging to history; dealing with history; literal, factual, authentic," and also "of historical importance" (early 15c.).
y-: perfective prefix, in yclept, etc.; a deliberate archaism, introduced by Spenser and his imitators, representing an authentic Middle English prefix y-, earlier i-, from Old English ge-, originally meaning "with, together" but later a completive or perfective element, from Proto-Germanic *ga- "together, with" (also a collective and intensive prefix), from PIE *kom "beside, near, by, with" (cognate with Sanskrit ja-, Latin com-, cum-; see com-). It is still living in German and Dutch ge-, and survives, disguised, in some English words (such as alike, aware, handiwork).

Among hundreds of Middle English words it formed are yfallen, yhacked ("completely hacked," probably now again useful), yknow, ymarried, ywrought.
ketchup (n.): 1711, said to be from Malay kichap, but probably not original to Malay. It might have come from Chinese koechiap "brine of fish," which, if authentic, perhaps is from the Chinese community in northern Vietnam [Terrien de Lacouperie, in "Babylonian and Oriental Record," 1889, 1890]. Catsup (earlier catchup, 1680s) is a failed attempt at Englishing, still in use in U.S., influenced by cat and sup.

Originally a fish sauce made from various plant juices, the word came to be used in English for a wide variety of spiced gravies and sauces; "Apicius Redivivus; or, the Cook's Oracle," by William Kitchiner, London, 1817, devotes 7 pages to recipes for different types of catsup (his book has 1 spelling of ketchup, 72 of catsup), including walnut, mushroom, oyster, cockle and mussel, tomata, white (vinegar and anchovies figure in it), cucumber, and pudding catsup. Chambers's Encyclopaedia (1870) lists mushroom, walnut, and tomato ketchup as "the three most esteemed kinds." Tomato ketchup emerged c. 1800 in U.S. and predominated from early 20c.
catawampus (adj.): also catawampous, cattywampus, catiwampus, etc. (see "Dictionary of American Slang" for more), American colloquial. First element perhaps from obsolete cater "to set or move diagonally" (see catty-cornered); second element perhaps related to Scottish wampish "to wriggle, twist, or swerve about." Or perhaps simply the sort of jocular pseudo-classical formation popular in the slang of those times, with the first element suggesting Greek kata-.

Earliest use seems to be in adverbial form, catawampusly (1834), expressing no certain meaning but adding intensity to the action: "utterly, completely; with avidity, fiercely, eagerly." It appears as a noun from 1843, as a name for an imaginary hobgoblin or fright, perhaps from influence of catamount. The adjective is attested from the 1840s as an intensive, but this is only in British lampoons of American speech and might not be authentic. It was used in the U.S. by 1864 in a sense of "askew, awry, wrong" and by 1873 (noted as a peculiarity of North Carolina speech) as "in a diagonal position, on a bias, crooked."