The Collaborative International Dictionary of English v.0.48:

Incautious \In*cau"tious\, a. [Pref. in- not + cautious: cf. L.
   incautus.]
   Not cautious; not circumspect; not attending to the
   circumstances on which safety and interest depend; heedless;
   careless; as, an incautious step; an incautious remark.
   [1913 Webster]

         You . . . incautious tread
         On fire with faithless embers overspread. --Francis.
   [1913 Webster]

         His rhetorical expressions may easily captivate any
         incautious reader.                       --Keill.

   Syn: Unwary; indiscreet; inconsiderate; imprudent; impolitic;
        careless; heedless; thoughtless. -- In*cau"tious*ly,
        adv. -- In*cau"tious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

incautious
    adj 1: lacking in caution; "an incautious remark"; "incautious
           talk" [ant: cautious]
    2: carelessly failing to exercise proper caution; "an incautious
       step sent her headlong down the stairs"

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "incautious":
   asleep, brash, brazen, brazenfaced, goofing off, hubristic,
   improvident, imprudent, impudent, indiscreet, injudicious,
   insolent, napping, nodding, off-guard, overbold, overcareless,
   overconfident, oversure, overweening, rash, sleeping, temerarious,
   unalert, uncautious, unchary, unguarded, unprepared, unready,
   unvigilant, unwary, unwatchful