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When the sleeves of an undershirt bulge out from the bottom of the sleeves of the shirt worn on top. See teespurt.

Also, a shirt that is crappy for any multitude of reasons: ugly, ill-fitting, poorly-made, smelly.
Just realized my favorite concert tee shirt becomes a blundershirt as soon as I put any other shirt over it.
by nrdesigner March 08, 2011
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