Deriving from the word fiend; It was created by ignorant methuen children who misinterpreted the word fiend. They had no idea what it meant or how to spell it so they came up with feen. For a definition, please see "Fiend".
Dumbass: Yo nigga you be feening on that shit.
Smarty Pants: Hey man, you mean fiend? and fiend isn't a verb?