something that's allowed is permissible. A film you should see is unmissable. Verily, English spelling is a thing of mystery.
If you hear the word a lot but it isn't in the dictionary, it means the dictionaries aren't keeping up with usage, which they should be. A word that's used a lot, and over a decent period of time (eg not just a trendy catchphrase from a TV show), and has a clear meaning, is a proper word.
Thanks folks, believe it or not, I consider myself as a brilliant speller, but that one passed me by, merely a spelling issue, but thanks for the response.
It's of no great help to you if you don't know Latin, of course, but - purely out of interest - English words ending in '-able' normally came originally from Latin words ending in '-abilis'. They came, in turn, from verbs ending in '-are' such as 'amare' = to love, 'mutare' = to change etc.
Similarly, our '-ible' words came from Latin verbs ending in '-ere' and '-ire'. Thus our 'admissible' from 'admittere' and 'audible' from 'audire'. 'Dismissible' is from 'dimittere'.