'radiation-proof' is a modern English compound formed by combining the noun 'radiation' and the suffix '-proof', where '-proof' is used to mean 'resistant to' or 'impervious to'.

'radiation' originates from Latin, specifically the word 'radiatio' (from 'radiare' meaning 'to emit rays'), and was adopted into English in the 19th century; the element '-proof' derives from Old English/Middle English 'prōf'/'proof' (originally meaning 'test' or 'trial') and later developed as a productive suffix (as in 'waterproof', 'bulletproof'), leading to compounds like 'radiation-proof' in modern English.

Initially, 'proof' related to testing or verification ('to prove'); over time it developed the additional meaning 'serving to protect against' as a suffix, so the compound now means 'protected against' or 'impervious to' radiation.