Not suitable or proper in the circumstances. Not in accordance with accepted standards of behavior.
From in- (not) + appropriate, which comes from Latin 'appropriatus' meaning made one's own or suitable. The negative form emerged in English in the mid-17th century as social codes became more refined and explicit.
What's considered inappropriate varies dramatically across cultures and time periods - showing ankles was scandalous in Victorian times, while today's casual Friday would have horrified 1950s office workers. Research shows that humor often works by violating social norms in a 'benign' way, walking the line of inappropriateness.
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