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1.
The sound made by either gas or objects passing through the lower human orifices especially when lubricated either artificailly or naturally.
That fart was a plappy one.

Her vagina has a certain plappiness to it, especially when entered from the rear.

by bazpartu January 11, 2007
 
2.
relating to the Prison Legal Assistance Project (aka PLAP) at Harvard Law School, an unbelievably awesome organization dedicated to representing prison inmates in their disciplinary hearings and parole hearings.

Not to be confused with the other definitions of plappy.
Don't worry, be PLAPpy
Hey, are you going to PLAPpy hour tonight?
by plap-ster November 23, 2010
 
3.
The combination of playful and happy.
She was plappy.

I feel plappy today. Hey!!!
by Steven Hendon January 26, 2008
 
4.
Adjective; having a gel or gelatinous texture or consistency. Similar to elderflower jelly or excess skin.
"That dessert was a little bit plappy".
by LemsipMax June 17, 2012