To disturb someone's confidence or composure; to confuse or embarrass someone by taking them by surprise.
From prefix dis- (opposite of) + concert (from Italian 'concertare,' to arrange together). First used in English in the 17th century to describe breaking someone's harmonious state of mind.
The word 'concert' in 'disconcert' has nothing to do with music—it comes from the Italian word meaning 'to arrange together.' So to disconcert someone is to literally 'un-arrange' their mental harmony, which is beautifully poetic for 'confuse them.'
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