back-formation

from WordNet (r) 3.0 (2006)

back-formation
    n 1: a word invented (usually unwittingly by subtracting an
         affix) on the assumption that a familiar word derives from
         it

from The Collaborative International Dictionary of English v.0.48

back-formation \back"-for*ma`tion\ n. (Linguistics)
   1. a word invented (usually unwittingly by subtracting an
      affix) on the assumption that a familiar word derives from
      it, such as emote from emotion.
      [WordNet 1.5 +PJC]

   2. the process of inventing a back-formation[1].
      [PJC]