In a way that is planned and done together with others; in a coordinated or unified manner.
From 'concerted' (past participle of 'concert,' meaning to plan together) plus the adverbial suffix '-ly.' The word 'concert' comes from Latin 'concertare,' meaning to bring into agreement.
When people act 'concertedly,' they're literally 'in concert' with each other—the word goes back to the idea of orchestrating or harmonizing efforts, just like musicians reading from the same score.
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