'dissimilarly' originates from English, formed from the adjective 'dissimilar' + the adverbial suffix '-ly', where 'dissimilar' originates from Latin 'dissimilis', with 'dis-' meaning 'not' and 'similis' meaning 'like'.

'dissimilar' came into English from Latin 'dissimilis' (via Romance languages/Middle English influences), and in Modern English the productive adverbial suffix '-ly' was added to form 'dissimilarly'.

Initially it meant 'not similar' (from Latin); the adverbial form has meant 'in a not-similar manner' and has retained that basic sense into modern usage.