Lettre de Cachet
(French). An arbitrary warrant of imprisonment; a letter folded and sealed with the king's cachet or little seal. These were secret instructions to the person addressed to proceed against someone named in the letter. The lieutenant-general of police kept an unlimited number of these instruments, and anyone, for a consideration, could obtain one, either to conceal a criminal or to incarcerate someone obnoxious. This power was abolished in the Revolution.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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