[ap-uh
l-kahrt] 
| 1. | a pushcart used by a vendor of apples. |
| 2. | upset the or 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. |
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.