'missile' originates from Latin, specifically the word 'missilis', related to 'mittere' meaning 'to send'. 'defense' originates from Old French 'defens', from Latin 'defendere', where 'de-' meant 'away/off' and 'fendere' (from 'fendere'/'fendere' roots) meant 'to strike/drive off'. The compound 'missile-defense' is a modern English formation (20th century) combining these elements to describe systems for defending against missiles.

'missile' passed from Latin 'missilis' into Middle French/Medieval Latin forms and then into Middle/Modern English as 'missile'. 'defense' came into English via Old French 'defens' (Middle English 'defence') from Latin 'defendere'. The phrase 'missile defense' emerged in the 20th century as military technology developed and the need to name interception systems arose.

Originally, 'missile' referred generally to anything that could be thrown or sent (a projectile) and 'defense' meant the action of defending; over time the combined term evolved into a technical phrase meaning specialized systems and measures to detect and intercept hostile missiles.