Cavendish, The Laws and Principles of Whist Stated and Explained…
“ The Grand Coup consists in throwing away a superfluous trump. At the first glance it appears impossible to have a superabundance of trumps; but cases sometimes happen where a player has a trump too many. To get rid of this trump—as by undertrumping a trick already trumped by your partner, or by trumping a trick which he has won, or which you know he may win—is to play the grand coup. ”
