Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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