applecart イディオム

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upset the applecart

ruin a plan or event by surprise or accident Everything was going well at the company until he came along and upset the apple cart.

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Don't upset the applecart

If you are advised not to upset the applecart, you are being told not to disturb the way things are done because it might ruin things.

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upset one's applecart|applecart|upset|upset the ap

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.