applecart

[ap-uh l-kahrt] /ˈæp əlˌkɑrt/
noun
1.
a pushcart used by a vendor of apples.
Idioms
2.
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
1780-90; apple + cart
British Dictionary definitions for upset the applecart

applecart

/ˈæpəlˌkɑːt/
noun
1.
a cart or barrow from which apples and other fruit are sold in the street
2.
upset the applecart, to spoil plans or arrangements
Idioms and Phrases with upset the applecart

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.