The Collaborative International Dictionary of English v.0.48:

Imprudent \Im*pru"dent\, a. [L. imprudens; pref. im- not +
   prudens prudent: cf. F. imprudent. See Prudent, and cf.
   Improvident.]
   Not prudent; wanting in prudence or discretion; indiscreet;
   injudicious; not attentive to consequence; improper. --
   Im*pru"dent*ly, adv.
   [1913 Webster]

         Her majesty took a great dislike at the imprudent
         behavior of many of the ministers and readers.
                                                  --Strype.
   [1913 Webster]

   Syn: Indiscreet; injudicious; incautious; ill-advised;
        unwise; heedless; careless; rash; negligent.
        [1913 Webster]

WordNet (r) 3.0 (2006):

imprudent
    adj 1: not prudent or wise; "very imprudent of her mother to
           encourage her in such silly romantic ideas"; "would be
           imprudent for a noneconomist to talk about the details of
           economic policy"- A.M.Schlesinger [ant: prudent]
    2: lacking wise self-restraint; "an imprudent remark"

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "imprudent":
   blanket, brash, brazen, brazenfaced, careless, casual, crazy,
   foolhardy, foolish, general, hasty, heedless, hubristic,
   ill-advised, ill-considered, ill-contrived, ill-devised,
   ill-gauged, ill-judged, impetuous, impolitic, improvident,
   impudent, impulsive, inadvisable, inane, incautious, inconsiderate,
   indiscreet, indiscriminate, indiscriminative, inexpedient,
   injudicious, insane, insensate, insensitive, insolent, irrational,
   irresponsible, mad, mindless, misadvised, misguided, myopic,
   nonjudgmental, overbold, overcareless, overconfident, oversure,
   overweening, perverse, promiscuous, rash, reasonless, reckless,
   senseless, shortsighted, silly, tactless, temerarious, thoughtless,
   unadvised, unchary, unconsidered, uncritical, uncriticizing,
   undemanding, undifferentiating, undiscerning, undiscreet,
   undiscriminating, undiscriminative, unexacting, unfastidious,
   unforeseeing, unmeticulous, unparticular, unreasonable,
   unreflecting, unreflective, unseeing, unselective, unsensible,
   unsound, unsubtle, untactful, unthinking, unthoughtful, unwary,
   unwise, wholesale, witless, wrong