Well, one of the primary obsessions of my youth(One that still lingers today) Was the Transformers...
Grimlock was the leader of the Dinobots(The T-Rex)...powerful, cunning in battle, and gruff though good at heart...if not cursed with a bit of a speech impediment
So a bunch of years back when I began my cyber-nerding in chat rooms, I needed a nick..."Rob" didn't seem cool enough, and I didn't wanna be real nerdy and pick "Optimus" or "Megatron" (Starscream sounded too much like a girl's name, according to my buds) so it went to my 4th favorite char: Grimmy.
Then as I went through some teenage years, got out of brainwashing sch...erm, catholic school, and developed a personality...I realized it fit me pretty well
So I shortened it to Grim (To be all 3l337 and stuff)...but that was taken, so I added "Man"
Ta-da.
-Grim