'pro-defense' is a compound of the prefix 'pro' (from Latin 'pro', meaning 'for' or 'in favor of') and the noun 'defense' (from Latin 'defendere', meaning 'to ward off' or 'to defend').

'defendere' in Latin passed into Old French as 'defendre' and into Middle English as 'defence/defense', eventually becoming the modern English 'defense' (US)/'defence' (UK); the prefix 'pro-' has been productive in English to form compounds meaning 'in favor of', producing 'pro-defense' in modern usage.

Initially the elements meant 'for' ('pro-') and 'to ward off' ('defendere'); combined in modern English they express the idea 'in favor of defense' or 'supporting defensive measures'.