'nondeductible' originates from the negative prefix 'non-' (from Latin 'non' meaning 'not') attached to 'deductible', which ultimately comes from Latin 'deductibilis' derived from 'deducere' where 'de-' meant 'away' and 'ducere' meant 'to lead'.

'deducere' in Latin gave rise to late Latin and Old French forms related to 'deduct-'; Middle English adopted 'deduct' (from Latin/Old French) and formed 'deductible' with the adjectival suffix '-ible'; in modern English the prefix 'non-' was added to create 'nondeductible'.

Originally the Latin root 'deducere' meant 'to lead away'; over time the sense shifted toward 'to subtract or take away' (as in subtracting an amount from a total). 'Deductible' came to mean 'able to be subtracted,' and 'nondeductible' now means 'not eligible to be subtracted,' especially for tax purposes.