One fairly common form of motivated root creation is echoic or onomatopoetic words. Hence cuckoo sounds like the call of the bird. Sometimes the creator attempts to summon connotations present in other words, like Tolkien's hobbit summons the image of a rabbit. Ex nihilo root creation has no logic behind it. Modification Modification is where an existing word is changed to form a new one.
Some people use the term folk etymology to describe a fanciful story behind a word's origin.
But etymologists use this term in a very specific, and different, way. Folk etymology is when an unfamiliar word is altered through common use to resemble a more familiar word. Hence, through the process of folk etymology cater corner becomes kitty corner . Functional Shift Words often shift their grammatical function over time. Nouns become verbs, verbs become nouns, nouns become adjectives, etc.