The Collaborative International Dictionary of English v.0.48:

Cachet \Cach"et\, n. [F. fr. cacher to hide.]
   A seal, as of a letter.
   [1913 Webster]

   Lettre de cachet [F.], a sealed letter, especially a letter
      or missive emanating from the sovereign; -- much used in
      France before the Revolution as an arbitrary order of
      imprisonment.
      [1913 Webster] Cachexia

WordNet (r) 3.0 (2006):

lettre de cachet
    n 1: a warrant formerly issued by a French king who could
         warrant imprisonment or death in a signed letter under his
         seal [syn: cachet, lettre de cachet]