'indecipherability' originates from English, formed by the negative prefix 'in-' (from Latin), the verb 'decipher' (English), and the noun-forming suffix '-ability' (from Latin '-abilitas'), where 'in-' meant 'not' and '-ability' indicated 'capability or suitability'.

'decipher' entered English from Old French (deschiffrer / déchiffrer), where 'chiffre' came via Medieval Latin and Old French ultimately from Arabic 'sifr' (meaning 'zero' or 'cipher'). Over time English formed 'decipher' (to decode), to which the prefix 'in-' and suffix '-ability' were later attached to produce 'indecipherability'.

Initially elements related to 'cipher' had the sense of 'number' or 'coded sign' (from Arabic 'sifr'), and 'decipher' meant 'to convert from cipher to ordinary language'; over time the compound 'indecipherability' came to mean the general quality of being unreadable or impossible to decode in modern usage.