Wiffin', as well as Periwinkle are the slang terms within the Mythos for someone who has explored outside the relative bounds of the Mythos for some reason. Such acts include, 'Killing the Slender Man', 'rewriting the Mythos to make Slender Man the hero', Powers, 'Sexual acts with the Slender Man', and other Out of Character concepts.
While Wiffin is frowned upon, it is simply a by-product of Chaotic Fiction itself. There will always be those who are interested in an idea, and seek to explore their own way with it. This in and of itself is fine, however when these people are brazen, or attempt to force their views on others, it tends to deride the Mythos, and cries of Wiffin' may be launched.
Also note, the penalty for wifin is a fee of 20 dollars. The fees collected suchly will be used to offset the cost of the Mary-Sue retirement home.
If anyone wishes to dispute an accusation of wiffin, the proper procedure is to give each party involved a hockey stick, a role of duct tape, and a live duck. If ducks are not available, geese may be used in a pinch. Then both parties must enter the Thunderdome.
It is very important to not confuse wiffin with wifi-n, common slang for the new generation of wireless technology characterized by MIMO capabilities.