Not allowed or acceptable, especially in a legal court where certain evidence cannot be used because it breaks the rules or is unreliable.
From 'in-' (not) plus 'admissible' (from Latin 'admittere,' to allow or let in). The legal sense developed in the 1600s as court systems formalized rules of evidence.
In a criminal trial, something a witness heard from someone else (hearsay) is usually inadmissible—even if it sounds important—because the court can't verify it's true, showing how law cares more about reliability than interesting gossip!
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