If it does not appear in any dictionary, then it is not 'permissible' in the sight of anyone who may have authority over your writing or anyone that you wish to respect your writing. Such formations are not by default, but by consensus: that is why only a limited number of the commoner and more euphonious bisyllabic -er comparatives have been generally accepted.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Mister MicawberIf it does not appear in any dictionary,Does that mean that I have to consult every dictionary in print? Sometimes you get different answers from different dictionaries.
Mister MicawberYou should make a reasonable effortWell, that's what I do do. I spend more time looking up words and ideas in dictionaries and encyclopedias than I spend writing my novel, which, after all, is just a sillly book.
Cool Breezean educated EnglishmanDiodn't Shakespeare use chiefest and perfectest?
Mister MicawberYes. Four hundred years ago.Chiefest? Perfectest? He did? Well, that settles it. I'm never speaking to him again.