Grundle (Aka-Trundle) being the area between your balls and your asshole.
Butter being...butter

Grundle Butter is greasy Sweat that gathers in the Grundle region, Semi-transparent in color with a refreshing odor.

SFM-Trundle Butter
"Why is Steves asshole wet?"

"Thats just his Grundle Butter."

"Poor Bastard"
by XPR December 1, 2004
Noun: A greasy semi-transparent fluid which accumulates in the area between the asshole and ball sack (known as the grundle)

Verb: Spread or cover (something) with grundle butter
Noun: "Why does that napkin smell like shit?"
"It's got grundle butter on it"

Verb: "Budget rental trucks pissed me off so bad I grundle buttered their doorknob while they were closed"
by SurvivingMike May 31, 2011
n. sweat from the balls and ass crack that congeal in the taint area.
'man, it's so hot today... i have so much grundle butter'
by osu985 February 20, 2009
An unfortunate gelatinous sweaty buildup on the grundle (which is the hair and area between your genitals and arse-hole).

I pulled off my jock and found a pat of grundle butter on it.
by madmark65 October 28, 2003
The left over cum on your dick and the cum that seeps back out of your partners twat after intercourse eventually mixing together and settling in your crotch.
I could have had a fish fry with all that grundle butter.
by Zac Hulsey January 7, 2006
A build up of sweat and unwashed buttcrack that results in a sticky, buttery poo-paste that may stain underwear or make it difficult to wipe successfully.
"Oh man, I keep wiping but there's just so much grundle butter! Maybe it's time to see a doctor..."
by slipperysl_hoe_pe November 30, 2018