'anti-annexationist' originates from modern English, specifically formed by combining the prefix 'anti-' (from Greek 'anti', meaning 'against') with 'annexation' (from Latin roots related to 'annexare'/'annexus', meaning 'to attach' or 'bound to') and the agentive suffix '-ist' (used to denote a person who holds a particular position).

'annex' derives from Latin 'annectere'/'annexus' (to join, bind to) and passed into English via Old French and Middle English; the modern compound 'anti-annexationist' developed in English by adding the prefix 'anti-' and the suffix '-ist' to describe someone opposed to annexation in political contexts.

Initially composed simply to mean 'against annexation', the term has retained that core sense and is used specifically in political and historical contexts to label individuals or groups opposing territorial annexation.