'pro-firearm' originates from a combination of elements in Latin and English: the prefix 'pro' (from Latin 'pro'), where 'pro' meant 'for', and the compound 'firearm' (English), itself formed from 'fire' (Old English 'fȳr') + 'arm' (Old English 'earm').

'pro-' has been used in modern English to form adjectives expressing support (e.g. 'pro-gun'); 'firearm' arose in Middle English as a compound of 'fire' + 'arm' referring to weapons that discharge projectiles; the compound 'pro-firearm' is a relatively recent modern formation by analogy with phrases like 'pro-gun'.

Initially the components meant 'for' (pro) and 'a weapon that discharges projectiles' (firearm); together they evolved to the current meaning 'in favor of the possession or use of firearms'.