swampsturbate

It refers to one masturbating with unwashed, infectiously dirty hands, the act resulting in a mixture of dirt and ejaculate which resembles to swamp.

Wife: Why don't you want to have sex?
Husband: Sorry honey, but on a 16 hour shift in the autoservice I had to swampsturbate in the lunch break. Couldn't hold it back anymore.
Wife: Gross!