'unfathomability' originates from English components: the prefix 'un-' (from Old English 'un-') meaning 'not', the root 'fathom' (from Old English 'fæðm') meaning 'embrace, outstretched arms' later used as a measure of depth, and the suffix '-ability' (from Latin '-abilitas') meaning 'capable of'.

'fathom' developed from Old English 'fæðm' (meaning 'embrace' or a measure of depth) into Middle English 'fathom'/'fathome', then into Modern English 'fathom'. The adjective 'fathomable' (able to be understood) and its negative 'unfathomable' formed later; the noun 'unfathomability' is derived from these components to denote the state of being not fathomable.

Initially, the root 'fæðm' referred to an embrace or a physical measure (depth). Over time 'fathom' acquired the metaphorical sense 'to understand', and thus 'fathomability' came to mean 'capability of being understood'; 'unfathomability' now means 'not capable of being understood' or 'inscrutability'.