Etymology
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fatuity (n.)

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

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

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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.

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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."
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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.

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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.

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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).
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infatuate (v.)

1530s, "turn (something) to foolishness, frustrate by making foolish," from Latin infatuatus, past participle of infatuare "make a fool of," from in- "in" (from PIE root *en "in") + fatuus "foolish" (see fatuous). Specific sense of "inspire (in someone) a foolish passion beyond control of reason" is from 1620s. Related: Infatuated; infatuating.

An infatuated person is so possessed by a misleading idea or passion that his thoughts and conduct are controlled by it and turned into folly. [Century Dictionary]
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fade (v.)

early 14c., "lose brightness, grow pale," from Old French fader "become weak, wilt, wither," from fade (adj.) "pale, weak; insipid, tasteless" (12c.), probably from Vulgar Latin *fatidus, which is said to be a blending of Latin fatuus "silly, tasteless" and vapidus "flat, flavorless." Related: Faded; fading. Of sounds, by 1819. Transitive sense from 1590s; in cinematography from 1918.

Adieu! adieu! thy plaintive anthem fades
  Past the near meadows, over the still stream,
    Up the hill-side; and now 'tis buried deep
      In the next valley-glades:
Was it a vision, or a waking dream?
  Fled is that music:" Do I wake or sleep?
[Keats, from "Ode to a Nightingale"]
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