Etymology
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fiery (adj.)
late 13c., "flaming, full of fire," from Middle English fier "fire" (see fire (n.)) + -y (2). The spelling is a relic of one of the attempts to render Old English "y" in fyr in a changing system of vowel sounds. Other Middle English spellings include firi, furi, fuiri, vuiri, feri. From c. 1400 as "blazing red." Of persons, from late 14c. Related: Fieriness. As adjectives Old English had fyrbære "fiery, fire-bearing;" fyren "of fire, fiery, on fire;" fyrenful; fyrhat "hot as fire."
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fire (n.)

Old English fyr "fire, a fire," from Proto-Germanic *fūr- (source also of Old Saxon fiur, Old Frisian fiur, Old Norse fürr, Middle Dutch and Dutch vuur, Old High German fiur, German Feuer "fire"), from PIE *perjos, from root *paewr- "fire." Current spelling is attested as early as 1200, but did not fully displace Middle English fier (preserved in fiery) until c. 1600.

PIE apparently had two roots for fire: *paewr- and *egni- (source of Latin ignis). The former was "inanimate," referring to fire as a substance, and the latter was "animate," referring to it as a living force (compare water (n.1)).

Brend child fuir fordredeþ ["The Proverbs of Hendyng," c. 1250]

English fire was applied to "ardent, burning" passions or feelings from mid-14c. Meaning "discharge of firearms, action of guns, etc." is from 1580s. To be on fire is from c. 1500 (in fire attested from c. 1400, as is on a flame "on fire"). To play with fire in the figurative sense "risk disaster, meddle carelessly or ignorantly with a dangerous matter" is by 1861, from the common warning to children. Phrase where's the fire?, said to one in an obvious hurry, is by 1917, American English.

Fire-bell is from 1620s; fire-alarm as a self-acting, mechanical device is from 1808 as a theoretical creation; practical versions began to appear in the early 1830s. Fire-escape (n.) is from 1788 (the original so-called was a sort of rope-ladder disguised as a small settee); fire-extinguisher is from 1826. A fire-bucket (1580s) carries water to a fire. Fire-house is from 1899; fire-hall from 1867, fire-station from 1828. Fire company "men for managing a fire-engine" is from 1744, American English. Fire brigade "firefighters organized in a body in a particular place" is from 1838. Fire department, usually a branch of local government, is from 1805. Fire-chief is from 1877; fire-ranger from 1887.

Symbolic fire and the sword is by c. 1600 (translating Latin flamma ferroque absumi); earlier yron and fyre (1560s), with suerd & flawme (mid-15c.), mid fure & mid here ("with fire and armed force"), c. 1200. Fire-breathing is from 1590s. To set the river on fire, "accomplish something surprising or remarkable" (usually with a negative and said of one considered foolish or incompetent) is by 1830, often with the name of a river, varying according to locality, but the original is set the Thames on fire (1796). The hypothetical feat was mentioned as the type of something impossibly difficult by 1720; it circulated as a theoretical possibility under some current models of chemistry c. 1792-95, which may have contributed to the rise of the expression.

[A]mong other fanciful modes of demonstrating the practicability of conducting the gas wherever it might be required, he anchored a small boat in the stream about 50 yards from the shore, to which he conveyed a pipe, having the end turned up so as to rise above the water, and forcing the gas through the pipe, lighted it just above the surface, observing to his friends "that he had now set the river on fire." ["On the Origins and Progress of Gas-lighting," in "Repertory of Patent Inventions," vol. III, London, 1827]
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Etna 
volcano in Sicily, from Latin Aetna, from an indigenous Sicilian language, *aith-na "the fiery one," from PIE *ai-dh-, from root *ai- (2) "to burn" (see edifice). Related: Etnean.
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mettlesome (adj.)

"full of spirit, fiery, courageous," 1660s, from mettle + -some (1). Related: Mettlesomely; mettlesomeness.

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Shadrach 
name of one of the three children delivered from the "fiery furnace" in Daniel iii.26.
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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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empyreal (adj.)
late 15c., "pertaining to the highest heaven," from Medieval Latin empyreus, from Greek empyros "fiery," from assimilated form of en (see en- (2)) + pyr "fire" (from PIE root *paewr- "fire") + -al (1). Confused by early writers with imperial.
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atrocity (n.)

1530s, "enormous wickedness," from French atrocité or directly from Latin atrocitatem (nominative atrocitas) "cruelty, fierceness, harshness," noun of quality from atrox "fierce, cruel, frightful," from PIE *atro-ek-, from root *ater- "fire" + root *okw- "to see;" thus "of fiery or threatening appearance." The meaning "atrocious deed" is from 1793.

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eczema (n.)
1753, from Greek ekzema, literally "something thrown out by heat," from ekzein "to boil over, break out," from ek "out" (see ex-) + zein "to boil," from PIE root *yes- "to boil, foam, bubble" (see yeast). Said to have been the name given by ancient physicians to "any fiery pustule on the skin" [Chambers' "Cyclopaedia"].
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*ater- 
*āter-, Proto-Indo-European root meaning "fire." It forms all or part of: atrabiliary; atrabilious; atrium; atrocious.

It is the hypothetical source of/evidence for its existence is provided by: Old Persian atar "fire;" Latin ater "black" ("blackened by fire"), atrox "frightful" ("of fiery or threatening appearance").
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