As you might guess, it's a third nipple.
I love it when you suck on my thripple.
by Lev Levowski February 15, 2006
A ladder-like extension fitted to the front, rear and sometimes sides of a haywagon, or other towed agricultural trailer so as to increase its capacity.
A thripple would be used by farmers to carry more produce than they would be able to if they just filled the bed of the trailer or wagon.
by Britgaffer September 20, 2009