'assailer' originates from English, specifically the word 'assail' (the agent form 'assailer'), where the elements come from Latin 'ad-' and 'salire' meaning 'toward' and 'to leap' (via Old French 'assaillir').

'assailer' changed from Old French 'assaillir' (to attack, literally 'to leap upon') through Middle English forms such as 'assailen'/'assaylen' and eventually produced the modern English verb 'assail' and the agent noun 'assailer'.

Initially it meant 'to leap upon; to attack' and over time the core sense remained 'to attack'; the noun developed to mean specifically 'one who attacks'.