After long periods of sitting and readjustment a mans nuts will catch on the boxer seam and be squeezed like a grape.
You know when you sit down for a long period of time and readjust a couple different times before standing up completely. Well a chair lobster is that pinching feeling when your ballsack somehow gets weirdly stuck between a layer of boxers. When you stand up, the ballsack gets slightly pinched by a fold in the fabric.

From Reddit user /u/moparornocar
by BurningRambo December 19, 2013
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