'formidability' is formed in English from the adjective 'formidable' plus the noun-forming suffix '-ity'; 'formidable' entered English from Old French 'formidable'.

'formidable' originated from Latin 'formidabilis', which came from 'formido' meaning 'fear' or 'dread'; the Latin form 'formidabilis' passed into Old French as 'formidable' and then into Middle/Modern English, where the noun 'formidability' was formed by adding '-ity'.

Originally associated directly with 'causing fear or dread' in Latin, the sense broadened in later use to include 'inspiring awe or respect' and eventually yielded the abstract noun meaning 'the quality of being formidable'.