'painkilling' originates from English as a compound of 'pain' + 'killing', where 'pain' came (via Old French 'peine') from Latin 'poena' meaning 'punishment, suffering', and 'kill' comes from Old English 'cwellan' meaning 'to strike dead, to kill'.

'pain' derived from Old French 'peine' (from Latin 'poena'), and 'kill' evolved from Old English 'cwellan' to Middle English forms such as 'killen' and then modern 'kill'; the present-participial form 'killing' combined with 'pain' in modern English to form the compound 'pain-killing' (ultimately written as 'painkilling').

Initially the parts literally conveyed 'killing pain' (i.e., acting on pain); over time the compound developed the specialized sense of 'relieving or suppressing pain' and is used adjectivally or as a noun for pain relief.