Underwear worn more than a 24 hour period of time by either a male or female. Underwear become Grunders after they are worn more than 24 hours or they are soiled and stained from urinary or anal leakage. The hallmark of turning your undies in to Grunders are those that are accentuated by skids and skid marks and mud marks.
Leroy was so behind in his laundry that he ran out of underwear. He ended up wearing the same pair for over two days. He finally did a load of whites after dropping skid marks in his Grunders.
Typically, a grunder (also known as; the georgia grunder) is performed by one female and many males. A large funnel is inserted into the anus of the female while she does a headstand, after which sweet tea is poured into the funnel. When the sweet tea is poured, and the funnel is removed...one of the males grabs the standard household plunger and places it over the anus of the female and plunges until the sweet tea is shot into the air like a fountain....after which the males participating open their mouths and try to drink from the fountain.
"Did you hear kelly is doin' a grunder tonight? Hot damn, where's my plunger!?"