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?"

See also: Cheerio, piss

See also:

Cheerio
pee in (someone's) Cheerios
piss in (someone's) Corn Flakes
piss on (someone's) chips
piss on (someone's) bonfire
cornflakes
piss in (one's) food
piss in (one's) soup
piss in (one's) cereal
shat

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