'pro-' originates from Latin, specifically the word 'pro', where 'pro' meant 'for' or 'in front of'; 'nuclear' originates from Latin, specifically the word 'nucleus', where 'nucleus' meant 'kernel' or 'core'.

'pro-nuclear' is formed in modern English by prefixing the Latin-derived prefix 'pro-' to the adjective 'nuclear' (which came into English via Latin 'nucleus' and French 'nucléaire' in the 19th century); the compound usage (hyphenated) became common in 20th-century political and technical discourse discussing nuclear power and weapons.

Initially, 'nucleus' referred to a small 'kernel' or 'core'; 'nuclear' later came to mean 'relating to the atomic nucleus' and by extension 'relating to nuclear power or weapons'; adding 'pro-' created the sense 'for' that modern compound expresses: 'in favor of nuclear (power/weapons)'.