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When a Cadbury's chocolate eclair toffee has been left in the car, and the inside has melted.

Thus, when you bite into the sweet, the chocolate is soft and can be squeezed out, just like a 'crapping bum'.
'Wow, Vic, i've just had the NICEST crappin' bum" Said Kate.
by Vic and Kate Stanley August 03, 2005