'prowar' originates from the combination of the prefix 'pro-' (from Latin 'pro', meaning 'for') and the English noun 'war' (from Old English and Old North French sources), where 'pro-' meant 'in favor of' and 'war' meant 'armed conflict'.

'war' developed from Old English (and earlier Germanic) forms and Old North French 'werre'; the productive English prefix 'pro-' has long been attached to nouns and adjectives to form compounds (e.g., 'pro-choice'), and the modern compound 'pro-war' arose by analogy and later appears informally as 'prowar'.

Initially the elements meant 'for' and 'armed conflict'; combined they have meant 'in favor of war' since formation, and that core meaning has remained stable though usage is chiefly political and sometimes pejorative.