The lava like substance that is found in the Grumble Volcano level of Mario Kart Wii. Can also be used to mean anything that would be undesirable to fall into.
When the green shell knocked me off the race track, i fell right into the Grumble.
Generally a girlfriend but then, all women "grumble". Therefore they are "grumble".
You can get high grade and low grade grumble.
Low grade grumble being dirty scallies...
The act of simultaneously pressing the left and right arrow keys on the keyboard whilst playing Bubble Trouble (on Miniclip). Usually appears to be a swift back and forth motion or sliding dance.
kid1: dude quit doing the grumble and play the game!!!
kid2: but you totally stole all my bubbles!