Balls that are unclean, sweaty, and hairy. Balls thats have a rank stench.
After I was done playing softball with the boys I had a bad case of musty nuts!
by BillC July 28, 2006
A scrotum that is infrequently washed and has a resultant pungent, moldy or mildewy odor similar to a semi-clean but oft-used public toilet.
I was gonna blow him until I got south of the border and got a whiff of his mustynuts.
by Gavinesq March 22, 2006