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):

impetuousness
    n 1: rash impulsiveness [syn: impetuousness, impetuosity]