piss in (someone's) Cheerios(redirected from pisses in their Cheerios)
piss in (someone's) Cheerios
rude slang To really upset, irritate, or disappoint someone. Sorry to piss 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 pissed in his Cheerios?"