piss in (someone's) Corn Flakes

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piss in (someone's) Corn Flakes

rude slang To really upset, irritate, or disappoint someone. Sorry to piss in your Corn Flakes, but you won't get any credits for the class unless you attend every single lecture. A: "Watch out, the boss is in a foul mood today." B: "Wow, I wonder who pissed in his Corn Flakes?"
See also: corn, Flake, piss