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):

impetuously
    adv 1: in an impulsive or impetuous way; without taking
           cautions; "he often acts impulsively and later regrets
           it" [syn: impetuously, impulsively]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "impetuously":
   a corps perdu, abruptly, bang, carelessly, dash, desperately,
   furiously, happen what may, hastily, head over heels, headfirst,
   headforemost, headlong, heedlessly, heels over head,
   helter-skelter, holus-bolus, hotfoot, hotheadedly, hurriedly,
   hurry-scurry, impulsively, like a flash, like a thunderbolt,
   like crazy, like mad, madly, of a sudden, on short notice,
   overeagerly, overenthusiastically, overzealously, plop, plump,
   plunk, pop, precipitantly, precipitately, precipitously, quickly,
   ramble-scramble, rashly, recklessly, sharp, slam-bang, slap,
   slap-bang, slapdash, smack, startlingly, sudden, suddenly,
   surprisingly, unawares, unexpectedly, wantonly, wildly,
   without notice, without warning