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:
Dummy
♠ A K 10 9 3
West
East
♠ J 5
♠ Q 2
Declarer
♠ 8 7 6 4
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. Idiot!
See also
Grosvenor Gambit
A purposeful defensive misplay, intended to bother declarer psychologically.