upset the applecart, to

To ruin or interfere with one's plans or goals. Look, he's not trying to upset the applecart—he just needs to meet later in the day now, that's all.

Spoil carefully laid plans, as in Now don't upset the applecart by revealing where we're going. This expression started out as upset the cart, used since Roman times to mean "spoil everything." The precise idiom dates from the late 1700s.

If someone or something upsets the applecart, they do something which causes trouble or which spoils a satisfactory situation. It will only upset the applecart and confuse the issue if the topic is raised too soon. Note: You can also say that someone or something overturns the applecart. She still has the power to overturn the applecart by the sheer force of her personality and vocabulary.

To ruin carefully laid plans. This metaphor, without the modifying “apple,” dates from Roman times. Both Plautus and Lucian used “You’ve upset the cart” to mean “You’ve spoiled everything.” Specifying “applecart” dates from the late eighteenth century, and the changed phrase is the one that survives. Grose’s Dictionary of the Vulgar Tongue (1796) suggests that “applecart” stood for the human body, and that the phrase meant to throw a person down, but that interpretation was either mistaken or the particular symbolism died out.