A person who has been arrested.
From arrest plus the suffix -ee, which indicates 'a person who receives the action.' This follows the pattern of words like employee (one who is employed) and trainee (one who is trained).
The -ee suffix is clever: it flips the perspective from the arrester (who arrests) to the arrestee (who is arrested), similar to how 'buyer' and 'seller' show different sides of the same transaction.
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