What does apple cart mean?

English as a Second Language English Teacher Explanations (ESL Tests)
1

English Language Tests, Intermediate level

ESL/EFL Test #424 [color=blue]“English Slang Idioms (163)”, question 1

“Did you hear that the warehouse workers are trying to form a union at this company? That is sure to upset the … around here,” Lisa told Linus.

(a) beanpole
(b) grocery
(c) water flow
(d) apple cart

English Language Tests, Intermediate level

ESL/EFL Test #424 [color=blue]“English Slang Idioms (163)”, answer 1

“Did you hear that the warehouse workers are trying to form a union at this company? That is sure to upset the apple cart around here,” Lisa told Linus.

Correct answer: (d) apple cart

Your answer was: [color=red]incorrect
“Did you hear that the warehouse workers are trying to form a union at this company? That is sure to upset the water flow around here,” Lisa told Linus.
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Hello,

Could you explain to me the meaning of apple cart?

2

An apple cart is a cart loaded with apples.
The idiom though is “to upset the apple cart”. Its meaning is explained here

3

Thank you.