Etymology
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irate (adj.)
1838, from Latin iratus "angry, enraged, violent, furious," past participle of irasci "grow angry," from ira "anger" (see ire).
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irascible (adj.)

late 14c., from Old French irascible (12c.) and directly from Late Latin irascibilis, from Latin irasci "be angry, be in a rage," from ira "anger" (see ire).

Irascible indicates quicker and more intense bursts of anger than irritable, and less powerful, lasting, or manifest bursts than passionate. [Century Dictionary]
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iracund (adj.)

"angry, inclined to wrath," 1707, from Late Latin iracundus, from ira "anger, wrath, rage, passion" (see ire (n.)). Related: Iracundulous (1765).

[T]he Severn is so mischievous and cholerick a river, and so often ruins the country with sudden inundations, since it rises in Wales, and consequently participates sometimes of the nature of that hasty, iracund people among whom 'tis born. [Thomas Browne, "Letters from the Dead to the Living," 1707]
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I.R.A. (2)
also IRA, initialism (acronym) for individual retirement account, attested from 1974.
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ire (n.)

c. 1300, from Old French ire "anger, wrath, violence" (11c.), from Latin ira "anger, wrath, rage, passion," from PIE root *eis- (1), forming various words denoting passion (source also of Greek hieros "filled with the divine, holy," oistros "gadfly," originally "thing causing madness;" Sanskrit esati "drives on," yasati "boils;" Avestan aesma "anger;" Lithuanian aistra "violent passion").

Old English irre in a similar sense is unrelated; it is from an adjective irre "wandering, straying, angry," which is cognate with Old Saxon irri "angry," Old High German irri "wandering, deranged," also "angry;" Gothic airzeis "astray," and Latin errare "wander, go astray, angry" (see err (v.)).

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Iranian (adj.)
1788, "of or pertaining to (ancient) Persia," from Iran + -ian. From 1839 in reference to the language. As a noun, "an inhabitant of Persia" (1792), later "the language of Persia" (1850). Iranic (adj.) is from 1847.
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