The Collaborative International Dictionary of English v.0.48:

Platitudinous \Plat`i*tu"di*nous\, a.
   Abounding in platitudes; of the nature of platitudes;
   uttering platitudes. -- Plat`i*tu"di*nous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

platitudinous
    adj 1: dull and tiresome but with pretensions of significance or
           originality; "bromidic sermons" [syn: bromidic,
           corny, platitudinal, platitudinous]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "platitudinous":
   aphoristic, axiomatic, back-number, banal, bewhiskered, bromidic,
   common, commonly known, commonplace, corny, crisp, current,
   cut-and-dried, epigrammatic, fade, familiar, formulaic,
   formulistic, fusty, gnomic, hackney, hackneyed, household,
   moth-eaten, musty, notorious, old hat, overworked, pithy, pointed,
   proverbial, public, pungent, sententious, set, square, stale,
   stereotyped, stock, succinct, talked-about, talked-of, terse,
   threadbare, timeworn, trite, truistic, universally admitted,
   universally recognized, unoriginal, warmed-over, well-kenned,
   well-known, well-recognized, well-understood, well-worn,
   widely known, worn, worn thin