Online Etymology Dictionary

fission (n.): 1819, "division of a cell or organism," from Latin fissionem (nominative fissio) "a breaking up, cleaving," from past participle stem of findere "to split" (see fissure). Cognate with Old English bitan "to bite." Nuclear physics sense is 1939. As a verb, from 1929.
fissure (n.): c. 1400, from Old French fissure (13c.) and directly from Latin fissura "a cleft," from root of findere "to split, cleave, separate, divide," from PIE *bhi-n-d-, from root *bheid- "to split" (source also of Sanskrit bhinadmi "I cleave," Old High German bizzan "to bite," Old English bita "a piece bitten off, morsel," Old Norse beita "to hunt with dogs," beita "pasture, food").
fist (n.): Old English fyst "fist, clenched hand," from West Germanic *fustiz (source also of Old Saxon fust, Old High German fust, Old Frisian fest, Middle Dutch vuust, Dutch vuist, German Faust), from Proto-Germanic *funhstiz, probably ultimately from PIE *penkwe- "five" (see five, and compare Old Church Slavonic pesti, Russian piasti "fist").

Meaning "a blow with the fist" is from 1767. Fist-fight "duel with the fists" is from c. 1600. As a verb, Old English had fystlian "to strike with the fist."
fistful (n.): "as much as a fist will hold," 1610s, from fist (n.) + -ful.
fistiana (n.): "anecdotes of pugilists; boxing lore," 1839, from fist (n.) + -iana.
fistic (adj.): "relating to or done with the fists," 1806, from fist (n.) + -ic. Long considered improper English ("Not in dignified use" - OED).
fisticuffs (n.): c. 1600, fisty cuffes, from fist (n.) + cuff (n.) "a blow" (see cuff (v.2)), with the form perhaps in imitation of handiwork. Related: Fisticuff.
fistula (n.): "long, narrow ulcer," late 14c., from Latin fistula "a pipe; ulcer," which is of uncertain origin. Related: Fistular; fistulous (Latin fistulosus "full of holes; tubular").
No certain etymology. The best comparison seems to be with festuca "stalk, straw" and maybe ferula "giant fennel" (if from *fesula): the forms of a "pipe" and a "stalk" are similar. The vacillation between fest- and fist- occurs within festuca itself, and might be dialectal, or allophonic within Latin. [de Vaan]
fit (n.1): 1680s, "process of fitting," from fit (v.). From 1823 as "the fitting of one thing to another;" 1831 as "the way something fits."
fit (n.2): "paroxysm, sudden attack" (as of anger), 1540s, probably via Middle English sense of "painful, exciting experience" (early 14c.), from Old English fitt "conflict, struggle," which is of uncertain origin, with no clear cognates outside English. Perhaps ultimately cognate with fit (adj.) on notion of "to meet." Meaning "sudden impulse toward activity or effort" is from 1580s. Phrase by fits and starts first attested 1610s (by fits is from 1580s).
fit (adj.): "suited to the circumstances, proper," mid-15c., of unknown origin, perhaps from Middle English noun fit "an adversary of equal power" (mid-13c.), which is perhaps connected to fit (n.1). In athletics, "in condition, properly trained for action," from 1869. Related: Fitter; fittest. Survival of the fittest (1867) coined by H. Spencer.
fit (v.): c. 1400, "to marshal or deploy (troops);" early 15c. as "be fitting or proper, be suitable," from fit (adj.) and perhaps in part from Scandinavian (compare Old Norse fitja "knit"). From 1580s as "be the right shape." Transitive sense of "provide with what is suitable" is from 1590s; that of "make fit or suitable, bring into corresponding form or condition" is from c. 1600. Related: Fitted; fitting. Fitted sheets is attested from 1948.
fit (n.3): part of a poem, Old English fitt, of unknown origin; perhaps related to fit (n.2).
fitful (adj.): used once by Shakespeare ("Life's fitful fever," "Macbeth," 1605) in sense of "characterized by fits," from fit (n.2) + -ful. then Revived in Romantic poetry late 18c. with a sense of "shifting, changing." Related: Fitfully (1792); fitfulness.
fitness (n.): 1570s, "state or quality of being suitable," from fit (adj.) + -ness. Meaning "state of being physically fit" is from 1935.
fitter (n.): 1650s, agent noun from fit (v.).
fitting (adj.): "proper, befitting, right," early 15c., present-participle adjective from fit (v.). Related: Fittingly.
fitting (n.): c. 1600, verbal noun from fit (v.). Meaning "action of fitting on a garment" is from 1900.
fitz (n.): Anglo-French fitz, from Old French fils, from Latin filius "son of" (see filial); used regularly in official rolls and hence the first element of many modern surnames; in later times used of illegitimate issue of royalty.
five (n.): Old English fif "five," from Proto-Germanic *fimfe (source also of Old Frisian fif, Old Saxon fif, Dutch vijf, Old Norse fimm, Old High German funf, Gothic fimf), from PIE *penkwe- (source also of Sanskrit panca, Greek pente, Latin quinque, Old Church Slavonic peti, Lithuanian penke, Old Welsh pimp). The lost *-m- is a regular development (see tooth).

Five-and-ten (Cent Store) is from 1880, American English, with reference to prices of goods for sale. Five-star (adj.) is from 1913 of hotels, 1945 of generals. Slang five-finger discount "theft" is from 1966. The original five-year plan was 1928 in the U.S.S.R. Five o'clock shadow attested by 1937.
[under picture of a pretty girl] "If I were a man I'd pay attention to that phrase '5 O'Clock Shadow.' It's that messy beard growth which appears prematurely about 5 P.M." [Advertisement for Gem razors and blades, Life," May 9, 1938]
fivefold (adv.): 1570s, from earlier use as an adjective, from Old English fiffeald (adj.); see five + -fold.
fiver (n.): 1843, "five-pound note," from five + -er.
fix (v.): late 14c., "set (one's eyes or mind) on something" (a figurative use), probably from Old French verb *fixer, from fixe "fixed," from Latin fixus "fixed, fast, immovable; established, settled," past participle adjective from figere "to fix, fasten, drive, thrust in; pierce through, transfix," also figurative, from PIE root *dhigw- "to stick, to fix" (see dike).

Sense of "fasten, attach" is c. 1400; that of "to make (colors, etc.) fast or permanent" is from 1660s. The meaning "settle, assign" evolved into "adjust, arrange" (1660s), then "repair" (1737). Sense of "tamper with" (a fight, a jury, etc.) is from 1790. As euphemism for "castrate a pet" it dates from 1930. Related: Fixed; fixing.
fix (n.): "position from which it is difficult to move," 1809, American English, from fix (v.). Meaning "dose of narcotic" is from 1934, shortened from fix-up (1867, originally in reference to liquor). Meaning "reliable indication of the position of a ship, plane, etc." (by reference to fixed positions) is from 1902.
fixable (adj.): late 15c., from fix (v.) + -able.
fixate (v.): 1885, "to fix, make stable," from fix (v.) + -ate. Meaning "to gaze upon" is from 1889. Psychological sense is from 1926, originally in Freudian theory, in this case perhaps a back-formation from fixation. Meaning "become fixed" is from 1888. Related: Fixated; fixating.
fixation (n.): late 14c., fixacion, an alchemical word, "action of reducing a volatile substance to a permanent bodily form," from Medieval Latin fixationem (nominative fixatio), noun of action from past participle stem of Latin fixare, frequentative of figere "to fix" (see fix (v.)). Meaning "condition of being fixed" is from 1630s. Used in the Freudian sense since 1910.
fixative (adj.): 1640s, from fix (v.) + -ative, suffix meaning "of or related to; tending to." As a noun, from 1870, "that which fixes."
fixed (adj.): late 14c., of stars, "unchangeable in position," past participle adjective from fix (v.). Related: fixedly (1590s). Fixed-income (n.) is from 1767.
fixer (n.): 1849, of chemicals, etc.; 1885 as a person who "makes things right;" agent noun from fix (v.). Fixer-upper is from 1967 as "that which repairs other things" (in an advertisement for a glue); by 1976 as a real-estate euphemism for "property that needs a lot of work."
fixings (n.): "apparatus," 1820, American English, from fixing "act of putting in order" (c. 1600), verbal noun from fix (v.). American English sense of "food, garnishing" is attested from 1839.
fixity (n.): 1660s in physics; general use from 1791; see fix (v.) + -ity.
fixture (n.): 1590s, "act of fixing," perhaps from fix (v.) on model of mixture, or from an assumed Latin *fixitatem. Meaning "anything fixed or securely fastened" is from 1812, an alteration of fixure (c. 1600).
fizgig (n.): "light, frivolous woman," 1520s, first element of uncertain origin, second element is Middle English gig "frivolous person" (see gig (n.1)).
fizz (v.): "make a hissing sound," 1660s, of imitative origin. Related: Fizzed; fizzing. The noun is recorded from 1812; meaning "effervescent drink" is from 1864, from the sound it makes.
fizzle (v.): 1530s, "to break wind without noise," probably altered from obsolete fist, from Middle English fisten "break wind" (see feisty) + frequentative suffix -le. Related: Fizzled; fizzling.

Meaning "make a noise as of a liquid or gas forced out a narrow aperture" is from 1859, "usually with special reference to the weakness and sudden diminution or cessation of such sound" [Century Dictionary], hence the figurative sense "prove a failure, stop abruptly after a more-or-less brilliant start." But this sense is earlier and dates to at least 1847 in American English college slang, along with the noun sense of "failure, fiasco" (1846), also originally U.S. college slang, "a failure in answering an examination by a professor." Barnhart says it is "not considered as derived from the verb." Halliwell ("Archaic and Provincial Words," 1846) has fizzle (v.) as "To do anything without noise," which might connect the college slang with the older word via some notion of mumbled and stifled performance:
In many colleges in the United States, this word is applied to a bad recitation, probably from the want of distinct articulation, which usually attends such performances. It is further explained in the Yale Banger, November 10, 1846: "This figure of a wounded snake is intended to represent what in technical language is termed a fizzle. The best judges have decided that to get just one third of the meaning right constitutes a perfect fizzle." [John Bartlett, "A Collection of College Words and Customs," Cambridge, 1851]
fizzy (adj.): 1885, from fizz + -y (2).
fjord (n.): 1670s, from Norwegian fiord, from Old Norse fjörðr, from North Germanic *ferthuz, from PIE *prtus, from root *per- (2) "to lead, pass over" (see port (n.1)). The etymological sense probably is "a going, a passage."
flab (n.): "fat, flabbiness," 1951, back-formation from flabby.
flabbergast (v.): 1772, flabbergasted, mentioned (with bored) in a magazine article that year as a new vogue word, of uncertain origin. Perhaps from some dialect (in 1823 flabbergast was noted as a Sussex word), perhaps ultimately an arbitrary formation alluding to flabby or flapper and aghast. "Like many other popular words expressing intensity of action, ... not separable into definite elements or traceable to a definite origin" [Century Dictionary]. Related: Flabbergasted; flabbergasting; flabbergastation.
flabbergasted (adj.): see flabbergast.
flabby (adj.): 1690s, regarded as a softened variant of flappy, which is recorded in the sense of "softly fleshy" from 1590s; see flap (n.). Related: Flabbily; flabbiness.
flaccid (adj.): 1610s, from French flaccide or directly from Latin flaccidus "flabby, pendulous, weak, drooping," from flaccus "flabby, flap-eared," which is of uncertain origin (OED suggests it's imitative). Related: Flaccidly; flacidness; flaccidity; flaccescency.
flack (n.): "publicity or press agent," 1945, also by that year as a verb, said to have been coined at show biz magazine "Variety" (but the first attested use is not in "Variety") and supposedly from name of Gene Flack, a movie agent, but influenced by flak. There was a Gene Flack who was an advertising executive in the U.S. during the 1940s, but he seems to have sold principally biscuits, not movies.
flag (n.1): "cloth ensign," late 15c., now in all modern Germanic languages (German Flagge, Dutch vlag, Danish flag, Swedish flagg, etc.) but apparently first recorded in English, of unknown origin, but likely connected to flag (v.1) or else an independent imitative formation "expressing the notion of something flapping in the wind" [OED]. A guess considered less likely is that it is from flag (n.2) on the notion of being square and flat.

Meaning "name and editorial information on a newspaper" is by 1956. U.S. Flag Day (1894) is in reference to the adopting of the Stars and Stripes by the Continental Congress on June 14, 1777.
flag (v.1): 1540s, "flap about loosely," probably a later variant of Middle English flakken, flacken "to flap, flutter" (late 14c.), which probably is from Old Norse flaka "to flicker, flutter, hang losse," perhaps imitative of something flapping lazily in the wind. Sense of "go limp, droop, become languid" is first recorded 1610s. Related: Flagged; flagging.
flag (n.2): "flat stone for paving," c. 1600, ultimately from Old Norse flaga "stone slab," from Proto-Germanic *flago- (see flake (n.)). Earlier in English as "piece cut from turf or sod" (mid-15c.), from Old Norse flag "spot where a piece of turf has been cut out," from flaga.
flag (n.3): plant growing in moist places, late 14c., "reed, rush," perhaps from Scandinavian (compare Danish flæg "yellow iris") or from Dutch flag; perhaps ultimately connected to flag (v.1) on notion of "fluttering in the breeze."
flag (v.2): 1875, "place a flag on or over," from flag (n.1). Meaning "designate as someone who will not be served more liquor," by 1980s, probably from use of flags to signal trains, etc., to halt, which led to a verb meaning "inform by means of signal flags" (1856, American English). Meaning "to mark so as to be easily found" is from 1934 (originally by means of paper tabs on files). Related: Flagged; flagging.
flagellant (n.): late 16c., "one who whips or scourges himself for religious discipline," from Latin flagellantem (nominative flagellans), present participle of flagellare "to scourge, lash" (see flagellum). There were notable outbreaks of it in 1260 and 1340s. As an adjective, "given to flagellation," 1880.