'undecipherably' originates from English, specifically the prefix 'un-' and the adjective 'decipherable', where 'un-' meant 'not' and 'decipher' ultimately comes from Old French/Medieval Latin roots related to Arabic 'sifr' (cipher/zero).

'decipher' changed from Old French/Medieval Latin forms (e.g. Old French 'decifr(er)' / Medieval Latin influenced forms) derived from a base related to Arabic 'sifr' ('cipher'), and in English became 'decipher'; later the adjective 'decipherable' formed, then the negative prefix produced 'undecipherable', and finally the adverbial suffix '-ly' produced 'undecipherably'.

Initially, components referred specifically to removing or relating to a 'cipher' (encoding); combined as 'undecipherable' it has long meant 'not able to be decoded,' and 'undecipherably' preserves that sense as 'in a way that cannot be decoded.'