an expression used to introduce bad or shocking news or gossip. Bill: I don't want to alarm you, but I see someone prowling around your car. Mary: Oh, goodness! I'll call the police!Bob: I don't want to upset you, but I have some bad news. Tom: Let me have it.
Fig. to mess up or ruin something. Tom really upset the apple cart by telling Mary the truth about Jane.I always knew he'd tell secrets and upset the apple cart.
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.