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1.
The quality of a beer mugs 'thumbrest', and its ability to allow one total control over the mug.
A beer mug with perfect or absolute grumption can be picked up with a single thumb. A large amount of beer mugs have good grumption, but the majority are on the lower scale of the grumption continuum.
by Double Dust January 20, 2008
 
2.
Someone with overly large testicles
"That bull has Grumption!"
by Dr Robertson May 05, 2007
 
3.
"May pure be the joys that surround you – and envelop you and carry you through"

That euphoric boost of energy you get for that last drive to the finish line...except for the whole event, sometimes in response to the sound of cowbells.
The TNT triathlete competed at Wildflower with sheer grumption.
by JB_070476 May 02, 2013