'inaptly' originates from Latin, specifically the word 'inaptus', where 'in-' meant 'not' and 'aptus' meant 'fit, suitable'.

'inapt' came into English from Latin 'inaptus' via Old French/Anglo-French forms (e.g. 'inapte') and Middle English 'inapt', and the adverb was formed by adding the suffix '-ly' to the adjective.

Initially it meant 'not fit or suitable' in Latin and early English, and over time it has retained that core meaning, now used to describe manner ('in a not fit or suitable way').