'pain-impervious' is a modern English compound combining the noun 'pain' and the adjective 'impervious'. 'Pain' ultimately comes from Latin 'poena' (via Old French 'peine'), meaning 'punishment' or 'suffering'. 'Impervious' is formed from the negative prefix 'im-' + 'pervious' (from Latin roots meaning 'allowing passage').

'impervious' entered English in the 17th century from Latin/French elements meaning 'not allowing passage'. 'pain-impervious' is a more recent, productive compound formed in Modern English by linking 'pain' with 'impervious' to mean 'not allowing pain to affect (one)'.

Originally, 'impervious' meant 'not allowing passage' (literal). Over time it acquired figurative senses such as 'not affected by' or 'immune to', and when combined with 'pain' it evolved to mean 'not affected by pain' (physical or figurative).