1. ruin or interfere with something; especially an established plan
I really don't like to upset the apple cart
When you 'upset the apple cart' you disturb someone's plans or upset the status quo. When you 'upset the apple cart' you disturb something which was planned or arranged. Similar in meaning to 'rock the boat' or 'to rain on someone's parade'. This is a useful expression you can find used in both social and professional contexts.