'indubitable' originates from Latin, specifically the word 'indubitabilis', where the prefix 'in-' meant 'not' and 'dubitabilis' related to 'dubitare' meaning 'to doubt'.

'indubitable' changed from the Latin word 'indubitabilis' into Late Latin and Middle English forms and was borrowed into English as 'indubitable' with little alteration to form.

Initially, it meant 'not to be doubted' in Latin, and over time it has retained essentially the same meaning in modern English: 'unquestionable'.