UPSET ONE'S APPLECART

{v. phr.}, {informal} To ruin a plan or what is being done, often by surprise or accident; change how things are or are being done, often unexpectedly; ruin or mix up another person's success or plan for success. * /John upset the other team's applecart by hitting a home run in the last inning and we won the game./ * /We are planning a surprise party for Bill, so don't let Mary upset the applecart by telling him before the party./ * /Frank thinks he is going to be the boss, but I'll upset his applecart the first chance I get./ Compare: ROCK THE BOAT.