Grundel is another word for chad: the region between the balls and the arse-hole; can be an erogenous zone for some.
Girl: I was fingering that part between his balls and his arse and there was shit there.
Everyone listening remained silent but later laughed among themselves:
'Eww! He had a shitty chad.'
When you measure your dick, you don't start at the arse-hole and include the chad; you risk getting shit on your measuring tape.
Grundles are brown cave-dwelling creatures from the My Little Pony movie. They look like monsters to most people, but, as they constantly reassure Baby Licketysplit and Spike the dragon, "Grundles good!" They are helpful, friendly creatures. Interestingly, Danny DeVito played the Grundle Leader in the movie.