Expression used for a male or female that has not had sexual interaction with another person in a long time. Much like in old western films, tumbleweeds are a symbol of barren and dry land. Your nether regions feel much like such a land when in this physical state. Ergo, pissing tumbleweeds.
"Dude, I haven't had any action in like, months. I'm pissing tumbleweeds."
"Are you kidding me? I went to a private Catholic high school. I was pissing tumbleweeds for four years!"