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

'unaccountable' evolved from the Old French 'acontable', which was adapted into Middle English as 'acountable', and eventually became 'unaccountable' with the addition of the prefix 'un-'.

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