The Collaborative International Dictionary of English v.0.48:

Ostentatious \Os`ten*ta"tious\, a.
   Fond of, or evincing, ostentation; unduly conspicuous;
   pretentious; boastful.
   [1913 Webster]

         Far from being ostentatious of the good you do.
                                                  --Dryden.
   [1913 Webster]

         The ostentatious professions of many years. --Macaulay.
   [1913 Webster] -- Os`ten*ta"tious*ly, adv. --
   Os`ten*ta"tious*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ostentatious
    adj 1: intended to attract notice and impress others; "an
           ostentatious sable coat" [syn: ostentatious,
           pretentious] [ant: unostentatious, unpretending,
           unpretentious]
    2: (of a display) tawdry or vulgar [syn: ostentatious,
       pretentious]