Preficize - Thick Dictionary

Preficize

verb
To add an incorrect prefix to a word to make a new word that is understandable, even though it is not necessarily a real word.

see sufficize

"un-" (opposite of, contrary to) + "fast" (characterized by quick motion, operation, or effect) = "unfast" (not a real word, but preficized so you know it means slow)