'nuclear-only' originates from Modern English compounding of the adjective 'nuclear' and the adjective/adverb 'only'; 'nuclear' ultimately comes from Latin 'nucleus' meaning 'kernel' and 'only' ultimately comes from Old English 'anlic' meaning 'single'.

'nuclear' came into English via Late Latin/New Latin 'nucleus' > Medieval/Modern Latin and then English 'nuclear'; 'only' developed from Old English 'anlic' to Middle English 'onliche'/'onli' and then modern 'only'. The compound 'nuclear-only' is a modern English formation used to indicate exclusivity to nuclear items or systems.

Initially 'nucleus' meant 'kernel' and 'nuclear' meant 'of or relating to a nucleus'; 'only' originally meant 'single/sole'. Over time 'nuclear' acquired the specific sense 'relating to the atomic nucleus' and 'only' retained the sense 'exclusive', so the compound came to mean 'exclusive to nuclear'.