"un" is a productive negation prefix. That means that it can freely be used just about anywhere it makes lexical sense to use it; there is no list of approved v

"un" is a productive negation prefix. That means that it can freely be used just about anywhere it makes lexical sense to use it; there is no list of approved verbs with which it may be used. Undesign is a perfectly valid word (and preferable to alternatives like dedesign in terms of aestheitics). Dictionaries cannot be expected to contain every variant of a word created by valid and systematic use of affixes.