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1.
The part of a grocery cart where parents put young children in diapers.
Did you see that guy putting pears in his cart? He didn't even put them in a plastic bag -- he just dumped them right there in the poop seat. I sure hope he washes them well when he gets home before he eats them.
by John 118 March 12, 2008
 
2.
The traditional name for a toilet on Nantucket Island, Massachusetts.

Popular use throughout New England began in the height of the old whaling days. When sailors returned from their long voyages, they were so excited to be able to sit down to defecate.

While the phrase is no longer used outside of Nantucket, it is still customary, particularly amongst the locals, to use the expression while on the island.
"Pardon me, is there a poop seat I can use?"

"The house has four bedrooms and two and half poop seats."

"Does anyone have to use the poop seat before we leave?"
by Jamie Panda July 20, 2006