An idiot coup is a defensive play that invites declarer to literally play the hand as if the defenders were idiots.
In the following layout, imagine that declarer needs 5 tricks from spades. He starts by leading a low spade toward dummy:
West follows with the J
as if he were a blockhead ostensibly splitting his spade honors. If the coup works, declarer will win this trick in dummy, cross back to his hand in a side suit, and lead a second spade toward dummy. West will play low, declarer will finesse, and East will win the Q.