'pro-annexationism' originates from Latin and Greek-derived elements: specifically the Latin prefix 'pro' (original word 'pro') where 'pro' meant 'for'; the Latin noun 'annexatio' (original word 'annexatio', from 'ad-' + 'nectere') where 'annexatio' meant 'a joining/attaching'; and the Greek suffix '-ismos' (via Latin '-ismus') where the suffix meant 'doctrine or practice'.

'pro-annexationism' developed by combining established English elements: Latin 'ad-' + 'nectere' ('to bind') produced Medieval/Latin forms like 'annectere' and later the Old French/Latin-derived English verb 'annex' and the noun 'annexation'; in modern English the prefix 'pro-' (meaning 'for') and the suffix '-ism' (denoting a doctrine) were attached to form 'pro-annexationism'.

Initially, the root idea (from Latin 'nectere'/'ad-') concerned 'tying/attaching' physically or legally; over time this evolved into the specific political/legal sense of 'adding or incorporating territory', and 'pro-annexationism' now means 'support for that act or policy'.