An intentional misplay that allows an opponent to gain a trick but is so ‘stupid’ that the opponent will never believe the misplay has occurred.
For example:
North cashes the ♠A and then leads the ♠J. East plays a ‘Grosvenor Coup’ by underplaying the two then the ten. South will never believe that East has failed to play the queen on the jack and so will play his king hoping for a 2-2 break.