The word 'undecipherable' is a modern English formation built upon the verb 'decipher'. The core concept, 'decipher', entered English in the 16th century from French, combining the notion of taking apart (dis-) a code or cipher (cifra). The suffix -able, of Latin origin, was then added to mean "capable of being deciphered." Finally, the Old English negative prefix un- was attached to reverse the meaning, creating an adjective that describes something which cannot be interpreted or made clear, such as illegible handwriting or an inscrutable message. The logic flows from the literal action of decoding to the abstract state of being impossible to decode.

Structure: un (not) + decipher (to decode) + able (capable of) - un: A prefix of Germanic origin (Old English) meaning "not," used to form negatives. - decipher: A verb from French déchiffrer, itself from Latin dis- (apart) + cifra (zero, cipher). In this compound, it functions as the core verb root. - able: A suffix from Latin -abilis, meaning "capable of, fit for," used to form adjectives.