'unresisting' originates in English as the negative prefix 'un-' combined with the present participle 'resisting' (from the verb 'resist'). 'Resist' comes from Latin 'resistere' meaning 'to stand back' or 'to withstand.'

'resist' entered English via Old French 'resister' from Latin 'resistere'; the adjective 'unresisting' is formed in Modern English by prefixing 'un-' to the present participle 'resisting', yielding the sense 'not resisting.'

Initially it meant simply 'not resisting' (literally lacking resistance); over time it has remained close to that sense but is also used figuratively to describe a passive or compliant attitude.