'unaccountable' originates from the prefix 'un-' meaning 'not' and the word 'accountable' from Old French 'acontable', which meant 'liable to be called to account'.

'acontable' transformed into the English word 'accountable', and with the addition of the prefix 'un-', it became 'unaccountable'.

Initially, it meant 'not liable to be called to account', but over time it evolved into its current meaning of 'not able to be explained or understood'.