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.
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.
type of Vietnamese soup, probably from French feu "fire" (see focus (n.)) "as in pot-au-feu, a stew of meat and vegetables of which the broth is drunk separately as a soup" [Ayto, "Diner's Dictionary"] which would have been acquired in Vietnamese during the French colonial period.
"lobby of a theater or opera house," 1859, from French foyer "green room, room for actors when not on stage," literally "fireplace," from Old French foier "furnace, stove, hearth, fireplace" (12c.), from Latin focarium, noun use of neuter of adjective focarius "having to do with the hearth," from focus "hearth, fireplace" (see focus (n.)).
also fusileer, 1670s, "soldier armed with a musket," from French fusilier "musket" (17c.), literally "piece of steel against which a flint strikes flame," from Old French fuisil, foisil "steel for striking fire; flint; whetstone; grindstone" (12c.), from Vulgar Latin *focilis (petra) "(stone) producing fire," from Latin focus "hearth," in Vulgar Latin "fire" (see focus (n.)). Retained by certain regiments of the British army that were formerly armed with fusils.
by 1994, from Italian focaccia, from Late Latin focacia, fem. of focacius, used of breads baked under the ashes, from Latin focus "hearth, fireplace" (see focus (n.)). Cognate with Spanish hogaza, Old French foace "griddle cake" (Modern French fouasse "a cake, bun"), Provençal fogassa.