A net search brings us to:

http://www.parlornotes.com/nonsense/cousinbilly.htm

There's a skeeter on my peter, whack it off!
There's a skeeter on my peter, whack it off!
There's a nuther on my bruther,
won't you whack that little muther,
There's a skeeter on my peter, whack it off!

Whack it off, whack it off!
Won't you whack it off for me?
It be the most fun you can have,
lest you find a nursin' calf,
Whack it off, whack it off!
Won't you whack it off for me?,

There's anodder on my cuzin,
Cain't you hear that sucker buzzin?
There's anodder on my cuzin,
Won't you whack it off for me?

Whack it off, whack it off!
Won't you whack it off for me?
It be the most fun you can have.

Sincerely,

Gargoyle