'indecipherably' originates from Modern English, specifically the adjective 'indecipherable' (formed from the verb 'decipher' with the prefix 'in-' meaning 'not' and the suffix '-able'), with the adverbial suffix '-ly' added.

'decipher' changed from Old French 'dechiffrer' (Modern French 'déchiffrer') and Medieval Latin forms related to 'decifrare'; 'cipher' itself traces back to Italian 'cifra' from Arabic 'sifr' meaning 'zero' (which developed into the sense 'code'). English formed 'indecipherable' by prefixing 'in-' to 'decipher' + '-able', and then '-ly' produced 'indecipherably'.

Initially the root 'cipher' (from Arabic 'sifr') was associated with 'zero' and later with coded writing; 'decipher' came to mean 'to remove the cipher' (to decode), and over time 'indecipherably' acquired the specific meaning 'in a way that cannot be decoded or read'.