Similar to the Blumpkin, however the oral sex received by the male while taking a dump is given by a person with no teeth. (Blumpkin + Gummer = Glumpkin)
Your toothless whore of a grandmother sure can give a mean Glumpkin!
OR....
I'm going to punch out your sister's teeth so she can give me a true Glumpkin.
"In Old Nan's stories the grumkins crafted magic things that could make a wish come true. Did I wish him dead? she wondered, before she remembered she was too old to believe in grumkin."