'impact-proof' is a Modern English compound formed from the noun 'impact' + the productive combining element '-proof' meaning 'resistant to' or 'protected against'.

'impact' entered English from Late Latin 'impactus' (past participle of 'impingere', 'to push into'/'to strike'), and '-proof' derives from Middle English 'proof' (Old French 'probe', from Latin 'probare' meaning 'to test'); the compound arose in modern usage by combining these elements.

Initially, elements meant 'a striking, pushing in' (for 'impact') and 'a test or protection' (for 'proof'); in the compound the sense shifted to 'resistant to striking or collision', i.e., 'able to withstand impacts'.