E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Imperturbable as Bertolde, i.e. not to be taken by surprise, thrown off your guard, or disconcerted at anything. Bertolde is the hero of a little jeu desprit in Italian prose, J. Cesare Croce. He is a comedian by profession, whom nothing astonishes, and is as much at his ease with kings and queens as with persons of his own rank and vocation.