From 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
From 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]