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1.
The "Poon" is known as a Womans Vagina as we know, so a "poon lagoon" is a Womans Bathroom.
Hey Sarah sorry i took so long in the poon lagoon.

or

Dood that chick droped a duce in the poon lagoon
by Joe Gadow January 12, 2006
 
2.
A place with lots of beautiful women.
Dude, this place is filled with hotties. It's like a poon lagoon!

I was just swimming in that poon lagoon.

There is a bevy of beautiful women here; it's a poon lagoon if you will.
by Special 'K' March 01, 2008
 
3.
Poon lagoon refers to the larger variety of the female vagina. The use of lagoon demonstrates a woman with an above average or catacylsmic poon.
That girl has a poon lagoon!
by Ollie Aynsley August 09, 2005