A Wallwalker or "Wall Walker" is a player of MMO games who pushes the boundaries of their virtual world to see how far the rabbit hole goes. Gamers who love to explore or find new/unusual ways around problems often refer to themselves as Wallwalkers.

The name was coined in the popular game World of Warcraft from an old exploit/feature (that was later patched) that allowed players to literally walk up walls when approached at the correct angle. This ability led explorers into new unseen game territory and aided in an advantage in PVP (battlegrounds).
It was made famous by a explorer who called himself Dopefish. Dopefish released several videos of exploits and unseen areas. His movie "Noggaholic" is probably the most widely known. At the beginning of one of his movies he said he was quiting the game and was quoted as saying "Never let the Wallwalkers fall". Shortly after the release of his movies, Wall walking was patched in the game, but will never be forgotten.

when you're hungover as shit after a spending a night drunk as a skunk

A wallwalker is a person who is constantly walking into walls, even when they are aware that the wall is there.

Someone new to an organisation who is so rubbish they would walk into a wall an still keep trying to go forward.

A booger that is sticky and attached to the side wall of a person’s nostril and unable to be blown or picked from the nose.