My first name is Matthew. My younger brother's first name is James. When we were little we didn't have a home console. We rented one every now and then from the movie store. When we did rent one we of course had to share. When we rented RPGs, often there would be only one save file so we'd always name our character MattJames. I'm the older brother so yeah my name always got priority. :p I can't remember which game it was (I think it may have been EARTHBOUND) but whatever it was, our save file name could only have 5 characters.
Thus "mattj" was bornded.
I'm pretty sure I've used mattj on every single game I've played since, and every forum I've ever been on. Now that my brother and I don't get to see each other every day its a nice reminder of lil Jimmy and those fun weekends playing SNES and 64 for like 48 hours straight.