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1.
n. a euphemism for masturbating, especially when done discreetly in public

Derived from the use of a small electronic handheld version of Solitaire, originally a single-player card game. Similarly, the person is performing a solo act of masturbation with his pocket rocket.
A man is sitting on a bench in a fitness room, staring at the ladies in tight shorts running on the treadmills.

One of the ladies is creeped out and tells a staff member:

"This man is playing pocket solitaire, and he's about to win!"
by idontdodrugsucreep January 22, 2012