The Collaborative International Dictionary of English v.0.48:

Impetuous \Im*pet"u*ous\, a. [F. impetueux, L. impetuosus. See
   Impetus.]
   1. Rushing with force and violence; moving with impetus;
      furious; forcible; violent; as, an impetuous wind; an
      impetuous torrent.
      [1913 Webster]

            Went pouring forward with impetuous speed. --Byron.
      [1913 Webster]

   2. Vehement in feeling; hasty; passionate; violent; as, a man
      of impetuous temper.
      [1913 Webster]

            The people, on their holidays,
            Impetuous, insolent, unquenchable.    --Milton.

   Syn: Forcible; rapid; hasty; precipitate; furious;
        boisterous; violent; raging; fierce; passionate. --
        Im*pet"u*ous*ly, adv. -- Im*pet"u*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

impetuous
    adj 1: characterized by undue haste and lack of thought or
           deliberation; "a hotheaded decision"; "liable to such
           impulsive acts as hugging strangers"; "an impetuous
           display of spending and gambling"; "madcap escapades";
           (`brainish' is archaic) [syn: hotheaded, impulsive,
           impetuous, madcap, tearaway(a), brainish]
    2: marked by violent force; "impetuous heaving waves"

Moby Thesaurus II by Grady Ward, 1.0:

225 Moby Thesaurus words for "impetuous":
   abrupt, accident-prone, active, acute, adrift, afloat, aggressive,
   alternating, amorphous, animated, antsy, antsy-pantsy, anxious,
   ardent, breakneck, breathless, brisk, brittle, capricious,
   careless, chafing, changeable, changeful, corruptible, deciduous,
   desperate, desultory, deviable, devil-may-care, dizzy, dying,
   dynamic, eager, eccentric, edgy, electrifying, energetic,
   enterprising, enthusiastic, ephemeral, erratic, evanescent,
   excited, fading, fast and loose, fervid, fickle, fierce, fiery,
   fitful, fleeting, flickering, flighty, flitting, fluctuating,
   fly-by-night, flying, forceful, forcible, fragile, frail, freakish,
   fretful, fretting, fugacious, fugitive, full of pep, furious,
   giddy, go-go, harum-scarum, hasty, headlong, hearty, hopped-up,
   hotheaded, hurried, impassioned, impatient, impermanent, impulsive,
   in a lather, in a stew, in a sweat, incisive, inconsistent,
   inconstant, indecisive, infirm, insubstantial, intense, irregular,
   irresolute, irresponsible, keen, kinetic, lively, living, lusty,
   mad, mazy, mercurial, mettlesome, momentary, moody, mortal,
   mutable, nerve-shattering, nondurable, nonpermanent, offhand,
   on edge, overeager, overenthusiastic, overhasty, overzealous,
   panting, passing, passionate, peppy, perishable, precipitant,
   precipitate, precipitous, quick, rambling, rash, reckless, restive,
   restless, robust, roving, rushing, scatterbrained, shapeless,
   shifting, shifty, shocking, short-lived, shuffling, simmering,
   slap-bang, slapdash, smacking, snappy, spanking, spasmodic, speedy,
   spineless, spirited, spontaneous, spur-of-the-moment, squirming,
   squirmy, startling, stormy, strenuous, strong, sudden, surprising,
   swift, take-charge, take-over, tempestuous, temporal, temporary,
   tornadic, transient, transitive, transitory, trenchant,
   unaccountable, unanticipated, uncertain, uncontrolled,
   undependable, undisciplined, undurable, uneasy, unenduring,
   unexpected, unfixed, unforeseen, unlooked-for, unpatient,
   unplanned, unpredictable, unpredicted, unpremeditated, unquiet,
   unreflective, unreliable, unrestrained, unsettled, unstable,
   unstable as water, unstaid, unsteadfast, unsteady, unthinking,
   vacillating, vagrant, variable, vehement, vibrant, vicissitudinary,
   vicissitudinous, vigorous, violent, vivacious, vivid, volatile,
   volcanic, wandering, wanton, wavering, wavery, wavy, wayward,
   whimsical, wild, wishy-washy, zestful, zesty, zippy