In a way that is not in accordance with accepted standards of behavior, correctness, or accuracy.
From Latin 'improprius' meaning 'not one's own' or 'inappropriate', with the prefix 'im-' (not) plus 'proprius' (one's own). The concept evolved to mean 'not suitable' or 'incorrect' by the 14th century.
The notion of 'proper' behavior varies dramatically across cultures and time periods - what's considered improperly dressed today might have been scandalous propriety in another era. Victorian ankles were as shocking as modern social media oversharing.
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