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):

ostentatiousness
    n 1: lack of elegance as a consequence of being pompous and
         puffed up with vanity [syn: ostentation,
         ostentatiousness, pomposity, pompousness,
         pretentiousness, puffiness, splashiness, inflation]