In a manner showing good taste, propriety, and dignity; with appropriate and respectable behavior.
From decorous (showing propriety and dignity) + -ly. Decorous comes from Latin decorus (becoming, proper, fitting). The term entered English in the 16th century and has maintained a formal, somewhat old-fashioned tone.
What's fascinating about 'decorously' is that it originally had nothing to do with decoration—it comes from Latin decorum (propriety), the same root as 'decorous.' But the similar-sounding 'decor' made people assume they were related, which demonstrates how our brains often connect words by sound rather than actual etymology.
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