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When you are being photographed, you place your hand underneath an article of clothing --- for example, your jacket --- and allow your thumb to stick out near your groin area, to resemble a penis.
"The chess team's yearbook photograph was ruined when Dimitry decided to pull a thumbcock."

Another example:

"The photograph of the wedding party was ruined when one of the groomsmen decided to thumbcock the photographer."
by NJRogerFeld September 08, 2009
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