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.