'unfathomable' originates from Old English, specifically the word 'fæðmian,' where 'fæðm' meant 'to embrace or surround.'

'fæðmian' transformed into the Middle English word 'fathom,' and eventually became the modern English word 'unfathomable' by adding the prefix 'un-' to denote negation.

Initially, it meant 'unable to measure the depth of,' but over time it evolved into its current meaning of 'incapable of being fully explored or understood.'