pissing in my cheerios
this is used as an alternative way to say that someone ruined the moment or your day.
"check out this girl I went out with last night, isn't she fine?"
"Ehhh, she's not all that"
"Damn bro, thanks for pissing in my cheerios!"
"Ehhh, she's not all that"
"Damn bro, thanks for pissing in my cheerios!"