'unreadably' originates from English, formed from the prefix 'un-' + the adjective 'readable' + the adverbial suffix '-ly', where 'un-' meant 'not' and '-ly' turned adjectives into adverbs.

'read' comes from Old English 'rǣdan' (to interpret, advise) which developed into Middle English 'reden'/'readen' and the modern verb 'read'; 'readable' was later formed by adding the suffix '-able' (from Latin '-abilis' via French/Latin influence) and 'unreadably' was formed in modern English by prefixing 'un-' and adding '-ly'.

Initially, the root 'read' had senses including 'interpret' or 'advise'; over time it narrowed to the modern sense 'to interpret written symbols', and 'unreadably' now means 'in a manner that cannot be read'.