Back Formations

Back formation is the term given to a word that comes into existence by removing the (supposed) affixes from a different word.

E.g. The verb "to Swindle" comes from the assumption that the noun "Swindler" is the agentive form of "swindle"

Essentially it's a word that comes into the lexicon because people assume it already exists (y is obviously a stem with x as the root, hence x is a word).