Latin for head of the dead, used by the old chemists to designate the residuum of chemicals, when all their volatile matters had escaped. Anything from which all that rendered it valuable has been taken away. Thus, a learned scholar paralysed is a mere caput mortuum of his former self. The French Directory, towards its close, was a mere caput mortuum of a governing body.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894