back-formation 
noun
1A word that is formed from an existing word which looks as though it is a derivative, typically by removal of a suffix (e.g. edit from editor).It was evidently a back-formation of gullibility, which in turn was an alteration of cullibility, ultimately from cull, meaning ‘a dupe’....
- The name ‘Troynovant’ is a back-formation from ‘Trinovantes’, the name of the powerful British tribe that lived north and east of London.
- The verb ‘to wrong’ is more common than the noun, and indeed the noun probably gets its enclitic meaning by back-formation from the verb.
1.1 [mass noun] The linguistic process by which back-formations develop.By a curious process of back-formation, a number of brand names, products and logos - Aeroplane jelly, Arnott's biscuits, Holden cars, Vegemite, the Qantas kangaroo - became national symbols in their own right....
- Rather, he says, ‘semi-auto’ is short for ‘semi-auto-loader,’ a translation from the German, and ‘semi-auto’ was lengthened by back-formation to ‘semi-automatic.’