The Collaborative International Dictionary of English v.0.48:

Decorous \Dec"or*ous\ (d[e^]k"[-o]r*[u^]s or d[-e]*k[=o]"r[u^]s;
   277), a. [L. dec[=o]rus, fr. decor comeliness, beauty; akin
   to decere. See Decent, and cf. Decorum.]
   Suitable to a character, or to the time, place, and occasion;
   marked with decorum; becoming; proper; seemly; befitting; as,
   a decorous speech; decorous behavior; a decorous dress for a
   judge.
   [1913 Webster]

         A decorous pretext the war.              --Motley.
   -- De*co"rous*ly, adv. -- De*co"rous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decorousness
    n 1: propriety in manners and conduct [syn: decorum,
         decorousness] [ant: indecorousness, indecorum]