not rolling his own 

The present participle is used to refer to the activity of masturbation in a larger completed sentence. Compare the word with the past tense wrang it out in the same passage: except I sure wrang it out a hunderd times thinkin about you.

The source is Brokeback Mountain. Jack did not roll his own. He did not did not jerk off alone. This sentence was omitted in the French translation of the short story.

with women, yeah, but Jesus H., ain't nothin like this. I
"sh*t no," said Jack, who had been riding more than bulls, not rolling his own. "You know that. Old Brokeback got us good and it sure ain't over. We got a work out what the f*ck we're goin a do now."

not rolling his own by John R. Gallup, Ph.D. [jrag] May 13, 2006