Grug is someone who is basically a Neanderthal

One who has a mania for music.

Someone who is doing something stupid or being a neanderthal.

The name of a caveman.

Rhymes with rug.

A failed attempt in a particularly spectacular fashion, specifically pertaining to sports or video games.

Someone who urgently requires a haircut.

Grug was a childrens book character crated by Ted Prior in a series of books popular in Australia throughout the late 80s and early 90s.

Grug has thus become a popular nickname Australian people in particular, including but not limited to those with the name Greg.

True grugs, however, resemble the book character in that their face or body is covered completely by hair.

Thus one who is in dire need of a haircut is a Grug. (or Cousin Itt.)

That feeling when you have nothing else to fucking say so you just say grug