'antiaggressively' is built from the prefix 'anti-' (originating in Greek 'antí'/'anti-' meaning 'against') combined with 'aggressive' (ultimately from Latin 'aggredi/aggess-' meaning 'to attack') and the English adverbial suffix '-ly' (Old English/Proto-Germanic element meaning 'in the manner of').

'aggressive' entered English via French (Old French/Modern French forms such as 'agressif') from Latin roots like 'aggredi'/'aggress-', then became English 'aggressive'; adding the prefix 'anti-' and the suffix '-ly' produced 'antiaggressively' in modern English usage.

Initially the Latin root 'aggredi' meant 'to attack'; over time the component 'aggressive' came to describe a tendency toward attack or force, while the prefix 'anti-' reversed that sense, and the combined modern form means 'in a manner opposing or not showing aggression'.