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focal (adj.)[focal 词源字典]: "of or pertaining to a focus," 1690s, from Modern Latin focalis; see focus (n.) + -al (1).[focal etymology, focal origin, 英语词源]
foci (n.): classically correct plural of focus (n.).
focus (n.): 1640s, "point of convergence," from Latin focus "hearth, fireplace" (also, figuratively, "home, family"), which is of unknown origin. Used in post-classical times for "fire" itself; taken by Kepler (1604) in a mathematical sense for "point of convergence," perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before Kepler, but it is not recorded). Introduced into English 1650s by Hobbes. Sense transfer to "center of activity or energy" is first recorded 1796.
focus (v.): 1775 in optics, "bring into focus" (transitive); 1807 in the figurative sense, from focus (n.). Intransitive use by 1864, originally in photography. Related: Focused; focusing; less commonly focussed; focussing.
fodder (n.): Old English fodder "food," especially "hay, straw, or other bulk food for cattle," from Proto-Germanic *fodram (cognates: Old Norse foðr, Middle Dutch voeder, Old High German fuotar, German Futter), from PIE *pa-trom, suffixed form of *pa- "to feed" (see food).
foe (n.): Old English gefea, gefa "foe, enemy, adversary in a blood feud" (the prefix denotes "mutuality"), from adjective fah "at feud, hostile," also "guilty, criminal," from Proto-Germanic *faihaz (cognates: Old High German fehan "to hate," Gothic faih "deception"), probably from PIE root *peig- (2) "evil-minded, treacherous, hostile" (cognates: Sanskrit pisunah "malicious," picacah "demon;" Greek pikros "bitter;" Latin piget "it irks, troubles, displeases," piger "reluctant, lazy;" Lithuanian piktas "wicked, angry," pekti "to blame"). Weaker sense of "adversary" is first recorded c. 1600.
foe-man (n.): also foeman, "active enemy," late Old English fah-man; see foe + man (n.).
foetal (adj.): see fetal; for spelling, see oe.
foetid (adj.): see fetid; for spelling, see oe.
foetus (n.): see fetus; for spelling, see oe.
fog (v.): 1590s (transitive), from fog (n.1). Intransitive use from 1849. Related: Fogged; fogging.
fog (n.1): "thick, obscuring mist," 1540s, a back-formation from foggy (which appeared about the same time) or from a Scandinavian source akin to Danish fog "spray, shower, snowdrift," Old Norse fjuk "drifting snow storm." Compare also Old English fuht, Dutch vocht, German Feucht "damp, moist." Figurative phrase in a fog "at a loss what to do" first recorded c. 1600. Fog-lights is from 1962.
fog (n.2): "long grass, second growth of grass after mowing," late 14c., probably of Scandinavian origin; compare Norwegian fogg "long grass in a moist hollow," Icelandic fuki "rotten sea grass." A connection to fog (n.1) via a notion of long grass growing in moist dells of northern Europe is tempting but not proven. Watkins suggests derivation from PIE *pu- (2) "to rot, decay" (see foul (adj.)).
fog-horn (n.): 1844, from fog (n.1) + horn (n.).
Fogarty: surname, from Old Irish fogartach "banished."
fogey (n.): also fogy, "an old, dull fellow," 1780, Scottish foggie, originally "army pensioner or veteran," perhaps connected to fogram (1772) "old-fashioned," also "old-fashioned person;" or from fog (n.2) in an obsolete senses of "moss," or from foggy "bloated, fat" (1520s), which perhaps is an extended sense of fog (n.2). Related: Fogeydom; fogeyish; fogeyism.
foggy (adj.): 1540s, of the air, "full of thick mist," perhaps from a Scandinavian source, or formed from fog (n.1) + -y (2). Foggy Bottom "U.S. Department of State," is from the name of a marshy region of Washington, D.C., where many federal buildings are (also with a suggestion of political murkiness) popularized 1947 by James Reston in "New York Times," but he said it had been used earlier by Edward Folliard of "The Washington Post."
foible (n.): 1640s, "weak point of a sword blade" (contrasted to forte), from French foible "a weak point, a weakness, failing," from noun use of Old French adjective feble "feeble" (see feeble). The spelling borrowed in English is obsolete in modern French, which uses faible. Extended sense of "weak point of character" is first recorded 1670s. Related: Foibles.
foie-gras (n.): 1818, French, short for pâté de foie gras (1827 in English), literally "pie of fat liver;" originally served in a pastry (as still in Alsace), the phrase now chiefly in English with reference to the filling. French foie "liver" is cognate with Italian fegato, from Latin *ficatum. For pâté see pate (n.2); for gras see grease (n.).
foil (v.1): c. 1300, foilen "to spoil a trace or scent by running over it" (more commonly defoilen), irregularly from Old French foler, fuler "trample on, injure, maim; ill-treat, deceive, get the better of" (13c., Modern French fouler), from Vulgar Latin *fullare "to clean cloth" (by treading on it), from Latin fullo "one who cleans cloth, a fuller," which is of unknown origin. Compare full (v.).

Hence, "to overthrow, defeat" (1540s; as a noun in this sense from late 15c.); "frustrate the efforts of" (1560s). Related: Foiled; foiling. Foiled again! as a cry of defeat and dismay is from at least 1847.