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A method of masturbation whereby the masturbator uses a traditional one-hand grip on the penis, while inserting the thumb from his other hand up his anus. Due to the slightly uncomfortable positioning, this practice can mean the participant takes a "long"time to complete the wank to satisfaction.

This method was apparently first seen performed in Scotland and in recognition of this, the Scottish word for Long, "Lang" is used.
I just had a lang wank, but boy, my arse is a little tender now and I can no longer bring myself to smell my thumb.
by The Artful Fudger December 12, 2010