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1.
A shit so big that it causes pain to the creator and causes plumbing issues to the toilet simultaneously. Often this can come naturally or be induced using copious amounts of ginger ale, caffeine, foods heavy in fiber and starch and sitting/laying in a folded position.
David's mega-poo induced vomiting and blocked the toilet.
by MegaPooInducer April 06, 2011
 
2.
1. Used to describe a poo in large form, or a large quantity of faeces.

2. Also used to describe someone named Meg as a grumpy poo-like person or their personality.
Meg is soo grumpy today, I wonder why.

I know, she's being a Megapoo.
by murmur1 October 31, 2011