Source: 
Bulletins
The Merrimac Coup is, I believe, named after one of the early ironclad military ships from the American Civil War. Presumably, the ship was often used on blockade duty, restricting the enemy’s entries, as it were.
When South opens a weak no trump (12-14 HCP), an immediate raise to 3NT is a reasonable gamble with the North cards. True, North has only 10 HCP, but the club suit will usually provide several winners, and the fast auction gives West a blind opening lead, which may prove to be to declarer’s advantage.
West leads the jack of spades to East’s ace. An automatic spade return, the choice of many players, will allow the contract home. Declarer wins the spade and knocks out the ace of clubs while the ace of diamonds sits in dummy as an entry to the established club winners.
The killing defence is for East to switch to the king of diamonds at trick two to knock out the dummy entry. East can then duck two rounds of clubs to restrict declarer to just two tricks from that suit. Though declarer gets three diamond winners, if the full deal is as shown, she is held to only eight tricks in all.
This spectacular play is a Merrimac Coup – the deliberate sacrifice of a high honour card with the object of knocking out a vital entry to either declarer’s or, more commonly, dummy’s hand.
If you find this play at the table you will be entitled to congratulation. Just yawn and say, ‘Oh, it was just a Merrimac Coup’, as though they are an every day occurrence for you