pee in (someone's) Cheerios(redirected from peeing in their Cheerios)
pee in (someone's) Cheerios
rude slang To really upset, irritate, or disappoint someone. Sorry to pee in your Cheerios, 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 peed in his Cheerios?"