pee in (someone's) 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?"
See also: Cheerio, pee
Farlex Dictionary of Idioms.
See also:
  • Cheerio
  • piss in (someone's) Cheerios
  • pee in (someone's) Corn Flakes
  • cornflakes
  • piss in (someone's) Corn Flakes
  • piss on (someone's) chips
  • piss on (someone's) bonfire
  • drive (someone) buggy
  • get someone's goat, to