Lacking in dignity, propriety, or good taste; behaving in a way that is unbecoming or impolite.
From 'de-' (reverse/lack) plus 'decorous' (from Latin 'decorus' meaning fitting or proper), creating an adjective that means the opposite of proper and dignified behavior.
This is a wonderfully formal way to call someone rude or undignified—it shows how English borrowed from Latin to create sophisticated vocabulary for disapproving of bad behavior.
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