'unoffending' is formed with the negative prefix 'un-' (from Old English 'un-') combined with 'offending', from the verb 'offend', which ultimately traces to Latin 'offendere' via Old French 'offendre'. In Latin, 'ob-' (variant 'of-') meant 'against' and 'fendere' meant 'to strike'.

'offendere' (Latin) > 'offendre' (Old French) > Middle English 'offenden'/'offend' > modern English 'offend' and its verbal/adjectival forms such as 'offending'; the prefix 'un-' was then attached to form 'unoffending'.

Originally 'offendere' meant 'to strike against' or 'to hit'; over time it came to mean 'to cause displeasure or hurt', and with the prefix 'un-' the sense shifted to 'not causing displeasure' as in 'unoffending'.