A cart for carrying apples. (Predominantly in phrases.).
"to upset a person's apple cart" and variants: (originally) to harm a person, to knock or cause a person to fall to the ground; (in later use chiefly) to spoil a person's plans. "to upset the apple cart" and variants: to knock over or let fall a thing; (now chiefly) to cause an upset or disturbance; to ruin an undertaking.
Mid 18th century. From apple + cart.