'annexationist' originates from the noun 'annexation' with the agentive suffix '-ist'. 'Annexation' comes from Medieval Latin 'annexatio', itself from Latin 'annexus' (past participle of 'annectere'), where the root 'annex-' meant 'bound to' or 'joined to', and the suffix '-ist' denotes 'a person who (does or supports)'.

'annexationist' changed from Latin 'annexare/annexatio' through Medieval Latin 'annexatio' and Old French forms such as 'annexion' into Middle English 'annexacioun' and modern English 'annexation', after which the English agentive suffix '-ist' was added to form 'annexationist'.

Initially related to the act of 'joining' or 'adding on' (literally 'binding to'), the term evolved to refer specifically to the political action or policy of adding territory and, by extension, to a person who advocates that policy ('supporter of annexation').