Etymology
Advertisement
ignis fatuus (n.)
"will o' the wisp, jack-o-lantern," 1560s, Medieval Latin, literally "foolish fire;" see igneous + fatuous. "It seems to have been formerly a common phenomenon; but is now exceedingly rare" [OED].
Related entries & more 
Advertisement
Jack-o'-lantern (n.)
also jack-o-lantern, jack-a-lantern, jackolantern, 1660s, "night-watchman;" 1670s as a local name for a will-o-the-wisp (Latin ignis fatuus), mainly attested in East Anglia but also in southwestern England. Literally "Jack of (with) the lantern;" see Jack + lantern. The extension to carved pumpkin lanterns is attested by 1834 in American English.
Related entries & more 
fatuity (n.)

1640s, from French fatuité (14c.), from Latin fatuitatem (nominative fatuitas) "foolishness, folly," from fatuus "foolish, insipid" (see fatuous).

Related entries & more 
ignivomous (adj.)
"vomiting fire," c. 1600, from Late Latin ignivomous, from Latin ignis "fire" (see igneous) + vomere "to vomit" (see vomit (n.)).
Related entries & more 
erysipelas (n.)

late 14c., skin disease also known as St. Anthony's Fire or ignis sacer, from Greek erysipelas, perhaps from erythros "red" (from PIE root *reudh- "red, ruddy") + pella "skin" (from PIE root *pel- (3) "skin, hide"). Related: Erysipelatous.

Related entries & more 
Advertisement
fatuous (adj.)

"foolish, stupid," 1530s, from Latin fatuus "foolish, insipid, silly;" which is of uncertain origin. Buck suggests originally "stricken" in the head. But de Vaan says from Proto-Italic *fatowo- "of speech," from PIE root *bha- (2) "to speak, tell, say."

[I]f we connect the fact that Fatuus is said to be an alternative name for Faunus, and that he predicted the future, and that this god is attested on an Etruscan mirror as Fatuvs in a clear oracular function (Weiss 2007b), we may venture a derivation from for 'to say' (Untermann 2000). The name of the god would then have come to be used pejoratively as 'silly'. [de Vaan]

Related: Fatuously; fatuousness.

Related entries & more 
fad (n.)
1834, "hobby, pet project" (adjective faddy is from 1824), of uncertain origin. Perhaps shortened from fiddle-faddle. Or perhaps from French fadaise "trifle, nonsense," which is ultimately from Latin fatuus "stupid." From 1881 as "fashion, craze," or as Century Dictionary has it, "trivial fancy adopted and pursued for a time with irrational zeal."
Related entries & more 
ignite (v.)

1660s (trans.), "kindle or set on fire, cause to burn," from Latin ignitus, past participle of ignire "set on fire, make red hot," from ignis "fire" (see igneous). Attested earlier as an adjective (1550s). Intransitive sense of "catch fire, begin to burn" is from 1818. Related: Ignited; igniting.

Related entries & more 
ignition (n.)

1610s, "act of heating to the point of combustion," from French ignition or directly from Medieval Latin ignitionem (nominative ignitio), noun of action from past-participle stem of Latin ignire "set on fire," from ignis "fire" (see igneous). Meaning "means of sparking a fire" (originally in a gun) is from 1881; meaning "means of sparking an internal combustion engine" is from 1906.

Related entries & more 
igneous (adj.)
1660s, "pertaining to or resembling fire," from Latin igneus "of fire, fiery; on fire; burning hot," figuratively "ardent, vehement," from ignis "fire, a fire," extended to "brightness, splendor, glow;" figuratively "rage, fury, passion," from PIE root *egni- "fire" (source also of Sanskrit agnih "fire, sacrificial fire," Old Church Slavonic ogni, Lithuanian ugnis "fire"). Geological meaning "produced by volcanic forces" is from 1791, originally in distinction from aqueous. Earlier in the sense "fiery" were ignean (1630s), ignic (1610s).
Related entries & more