The Collaborative International Dictionary of English v.0.48:

De rigueur \De ri`gueur"\ [F. See 2d Rigor.]
   According to strictness (of etiquette, rule, or the like);
   obligatory; strictly required.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

de rigueur
    adj 1: required by etiquette or usage or fashion; "instruction
           as to when and where a silk hat is de rigueur"

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "de rigueur":
   accepted, acknowledged, admitted, approved, au fait, becoming,
   being done, binding, comme il faut, conformable, conventional,
   correct, customary, decent, decorous, done, formal, imperative,
   imperious, mandatory, meet, must, necessary, nice, obligatory,
   orthodox, peremptory, proper, received, recognized, required,
   right, seemly, traditional