[Bar-told ]. 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 d'esprit 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.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894