back-formation

Short definition from Wiktionary

Noun

1. 1
   The process by which a new word is formed from an older word by interpreting the former as a derivative of the latter, often by removing a morpheme (real or perceived) from the older word, such as the verb burgle, formed by removing -ar (perceived as an agent-noun suffix) from burglar.
2. 2
   A word created in this way.
3. 3
   Alternative form of back-formation.