shit in someone's Cheerios

From the breakfast food Cheerios. The term comes from the notion that defecating in someone's Cheerios would ruin their day.

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shit in someone's Cheerios (third-person singular simple present shits in someone's Cheerios, present participle shitting in someone's Cheerios, simple past and past participle shit in someone's Cheerios or shitted in someone's Cheerios or shat in someone's Cheerios)

1. (slang, vulgar, idiomatic) To ruin one's day.

   That woman shit in her husband's Cheerios when she divorced him on their anniversary.