When the excrement in one's anus cannot be contained any longer and it starts pushing open the sphincter. Probably related to childbirth where the top of the baby's head (the crown) is first visable as it emerges from the vagina.
"OMG I'm crowning so bad it's like I've got a turtle neck hanging from my ass..."
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Bill:Hey what meeting are going to tonight?
Bob: Gucci Meeting, itรขยยs the best meeting in OC!