Vader has only got one ball was a popular drinking and marching song among Rebel soldiers and pilots. In the Galactic Empire, singing this song or listening to it was tantamount to high treason. It was sung on a tune of a traditional Jawa folk song, "Dink Dink".
Land of soap and water,
Vader's having a bath.
Mothma peeps through a keyhole,
Having a good laugh...
Vader has only got one ball
Tarkin has two, but very small,
Jabba has balls of rubber,
And Palpatinehas got no balls at all!
Vader has only got one ball,
The other is on an adobe wall,
Watto, who's always blotto,
Cut it off when he was small.
He threw it into a sarlacc's face,
But missed, and it got into deep space.
The cave-slug got out his space mug,
And had some bollocks and some oratays.
The "Mother vs Watto" controversy Edit
Originally, the common Rebel soldiers knew nothing about Vader's backstory and didn't know who is Watto. The first variant included the line "His mother, the dirty bugger" instead of the line mentioning Watto. When Luke and Leia learned their father's full story, they insisted on changing the line to its modern variant.