Synonyms of 'indecipherable'
illegibleDictionary, unintelligible, indistinguishable
Her writing is virtually indecipherable.
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Unintelligible language is impossible to understand, for example because it is not written or pronounced clearly, or because its meaning is confused or complicated.
He muttered something unintelligible.
pages inscribed with unintelligible characters
Thesaurus for indecipherable from the Collins English Thesaurus
If writing or speech is indecipherable , you cannot understand what the words are.