Synonyms for Decorous (same or very similar meaning)
WordNet sense 1 (according with custom or propriety):
becoming, comely, comme il faut, decent, seemly
WordNet sense 2 (marked by suitability or rightness or appropriateness):
proper
WordNet sense 3 (characterized by dignity and propriety):
sedate, staid
From the ODE community, based on WordNet
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