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.