A Grungina is a vagina-like fold of skin that appears on Grun (or Grune), due to two meaty appendages coming into contact (ie the thighs) or due to one meaty appendage folding in on itself (ie the elbows).
Grungina is a well-documented medical condition, and is not to be sneered at. Please take it seriously, as one day, you could be a victim.
Shit Grune, you've got Grungina Elbow again!
Grune, your ass crack's making a right Grungina!
How the fuck have you managed to get grungina in the back of your neck?!