Wiktionary
unspellable
a. Difficult or impossible to spell.
Usage examples of "unspellable".
It was slightly open, as though it froze in the middle of an unspellable word.
As the combined meaning began to come through, I screamed an unspellable syllable, spun on my heel and sprinted as fast as I could to my own cottage next door, kicked the back door open, and raced inside.