'countermissile' originates from English compounding of the prefix 'counter-' and the noun 'missile', where 'counter-' meant 'against' and 'missile' comes from Latin 'missilis' meaning 'that may be thrown' or 'projectile'.

'missile' comes into English via Old French and directly from Latin 'missilis' (from 'mittere', 'to send'), while the prefix 'counter-' derives from Old French 'contre' and ultimately Latin 'contra' meaning 'against'; these elements were combined in modern English to form 'countermissile' (compound formation in 20th-century military usage).

Initially the elements meant 'against' + 'projectile'; over time the compound came to specifically denote a weapon designed to counter (intercept or destroy) another missile, especially in the context of missile defense.