Troll Under a Bridge

What does this mean? I was reading a thread and someone effectively claimed to be a troll under a bridge in response to being called a know-it-all by others on the forum. He posted a photo of a sculpture here in Seattle of a giant under a bridge with a volkswagen in one hand. Only after I looked at the picture's file name did I realize "troll under a bridge" meant something.

It's from a story -- "Three Billy Goats Gruff." It is several places on the Web.