'De rigueur' definitions:

Definition of 'de rigueur'

From: WordNet
adjective
Required by etiquette or usage or fashion; "instruction as to when and where a silk hat is de rigueur"

Definition of 'De rigueur'

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