A word invented (usually unwittingly by subtracting an affix) on the assumption that a familiar word derives from it WordNet 3.0 & Open English WordNet

In etymology, back-formation is the process or result of creating a new word via inflection, typically by removing or substituting actual or supposed affixes from a lexical item, in a way that expands the number of lexemes associated with the corresponding root word. Wikipedia

The process of creating a new lexeme by removing or affixes Wikipedia Disambiguation

Creating a word by removing actual or supposed affixes, such as "kempt" from "unkempt" Wikidata

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. Wiktionary