E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Coup de Théâtre.
An unforeseen or unexpected turn in a drama to produce a sensational effect. In ordinary life, something planned for effect. Burke and his dagger was meant for a coup de théâtre, but it was turned into farce by a little ready wit. (See DAGGER-SCENE.)