E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Lettre de Cachet (French).
An arbitrary warrant of imprisonment; a letter folded and sealed with the kings 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.