'indubitably' originates from Latin, specifically the word 'indubitabilis', where 'in-' meant 'not' and 'dubitare' meant 'to doubt'.

'indubitably' changed from the Latin adjective 'indubitabilis' into English via Late Latin/Medieval Latin and the derivative English adjective 'indubitable', with the adverb formed by adding the suffix '-ly' to make modern 'indubitably'.

Initially, it meant 'not able to be doubted' (i.e., beyond doubt); over time it has come to be used as an adverb meaning 'without doubt; unquestionably'.