Nickname for a large VERY heavily muscled man, more likely a powerlifter or strongman than a bodybuilder. Usually used when the guy looks the part-- sweat-soaked T-shirt, lifting belt, chalk dust, etc. Calling a woman strength athlete "Gruntus" is usually an insult, so choose wisely.
GymRat: "I'm gonna do some deadlifts, got any chalk?"
Lifter: "Naah, go ask Gruntus over at the power rack."
GymRat: "No way, man, he looks like he'd snap me like a toothpick!"
"Grungus" is a word used to describe filth. It is both an adjective and a noun. Things that are dirty/filthy are "grungus" or "covered in grungus." The word is more than this: it is the embodiment of disgust and repulsion, and is typically used hyperbolically.
"Sis wtf did you cook in here, the kitchen is covered in grungus."
or
(reacting to something gross) "That shit's grungus, bro."