a pushcart used by a vendor of apples.
upset the/someone's applecart, to ruin plans or arrangements; spoil something: He was making a fantastic profit until a competitor upset the applecart by cutting prices.
Origin of applecart
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a cart or barrow from which apples and other fruit are sold in the street
upset the applecart to spoil plans or arrangements
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
upset the applecart
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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.