Etymology
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impetuous (adj.)
late 14c., "hot-tempered, fierce;" late 15c., "done or given with a rush of force," from Old French impetuos (13c., Modern French impétueux) and directly from Late Latin impetuosus "impetuous, violent" (source also of Spanish and Italian impetuoso), from Latin impetus "attack" (see impetus). Related: Impetuously; impetuousness.
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impetuosity (n.)
early 15c., "violent movement, rushing," from Old French impetuosité (13c.) and directly from Medieval Latin impetuositatem (nominative impetuositas), from Late Latin impetuosus "impetuous, violent" (see impetuous).
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*pet- 

Also petə-, Proto-Indo-European root meaning "to rush, to fly." 

It forms all or part of: accipiter; appetence; appetite; apterous; apteryx; archaeopteryx; asymptote; centripetal; Coleoptera; compete; competent; eurypterid; feather; helicopter; hippopotamus; Hymenoptera; impetigo; impetuous; impetus; iopterous; Lepidoptera; ornithopter; panache; panne; pen (n.1) "writing implement;" pennon; peripeteia; perpetual; perpetuity; petition; petulance; petulant; pin; pinion; pinnacle; pinnate; pinniped; potamo-; potamology; propitiation; propitious; ptero-; pterodactyl; ptomaine; ptosis; repeat; symptom.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pattram "wing, feather, leaf," patara- "flying, fleeting;" Hittite pittar "wing;" Greek piptein "to fall," potamos "river, rushing water," pteron, pteryx "feather, wing," ptilon "soft feathers, down, plume;" Latin petere "to attack, assail; seek, strive after; ask for, beg; demand, require," penna "feather, wing;" Old Norse fjöðr, Old English feðer "feather;" Old Church Slavonic pero "feather;" Old Welsh eterin "bird."

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vehement (adj.)

early 15c., from Old French vehement, veement "impetuous, ardent" (12c.), from Latin vehementem (nominative vehemens) "impetuous, eager, violent, furious, ardent, carried away," perhaps [Barnhart] from a lost present middle participle of vehere "to carry" (from PIE root *wegh- "to go, move, transport in a vehicle"). The other theory is that it represents vehe- "lacking, wanting" + mens "mind." Related: Vehemently.

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Rahab 
name of a Biblical monster, from Hebrew rahab, literally "storming, against, impetuous," from rahabh "he stormed against" (compare Arabic rahiba "he feared, was alarmed").
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heady (adj.)
late 14c., "headstrong, hasty, impetuous," from head (n.) + adj. suffix -y (2). First recorded 1570s in sense of "apt to go to the head." Related: Headily; headiness.
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hasty (adj.)
mid-14c., "early; demanding haste, urgent; quick-tempered, angry;" late 14c. "speedy, swift, quick," by 1500s, from haste (n.) + -y (2); replacing or nativizing earlier hastif (c. 1300) "eager, impetuous," from Old French hastif "speedy, rapid; forward, advanced; rash, impetuous" (12c., Modern French hâtif), from haste (see haste (n.)). Meaning "requiring haste" is late 14c. (this is the sense in hasty-pudding, 1590s, so called because it was made quickly); that of "eager, rash" is from early 15c. Related: Hastiness. Old French also had a form hasti (for loss of terminal -f, compare joli/jolif, etc.), which may have influenced the form of the English word.
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fervent (adj.)
mid-14c., from Old French fervent "fervent, ardent" (12c.), from Latin ferventem (nominative fervens) "boiling, hot, glowing," figuratively "violent, impetuous, furious," present participle of fervere "to boil, glow," from PIE root *bhreu- "to boil, bubble, effervesce, burn." The figurative sense of "impassioned" is first attested c. 1400. Related: Fervency; fervently.
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testy (adj.)
early 15c., "impetuous, rash," altered from Middle English testif "headstrong" (late 14c.), from Anglo-French testif, Old French testu (Modern French têtu) "stubborn, headstrong, obstinate," literally "heady," from teste "head" (see tete). Meaning "easily irritated, irascible" is first recorded 1520s. Related: Testily; testiness.
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pell-mell (adv.)

"confusedly; in an impetuous rush; with indiscriminate violence, energy, or eagerness," 1570s, from French pêle-mêle, from Old French pesle mesle (12c.), apparently a jingling rhyme on the second element, which is from the stem of the verb mesler "to mix, mingle" (see meddle). The phonetic form pelly melly is attested in English from mid-15c.

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