The said "grampy panties" are similar to tidy whities; the panty line is easily discerned through any loose clothing, like basketball shorts, especially if the clothing is a light color. They ride up and make uncomfortable and particulary ass-noying wedgies that, when picked, decrease the overall manly dignity to a measly amount.
Those who wear grampy panties are subject to immediate mockery when the panty line is spotted. If you are discovered as a grampy-wearer, head to the nearest bathroom and remove the grampies and place them in the toilet, so that no-one can make fun of you any longer. Just make sure your name is not written on the tag.
"Dang, check out his grampies!"
"NUMBER 7 IS SEEN SPORTING WHITE SHORTS, EMBARRASSINGLY REVEALING HIS EXTRA-LARGE GRAMPY PANTIES."