'torpedo-proof' originates from English as a compound of 'torpedo' and 'proof'. 'torpedo' itself ultimately comes from Latin 'torpidō' (from 'torpere') where 'torpere' meant 'to be numb or stiff', and 'proof' comes from Old English/Old French roots meaning 'tested' or 'resistant'.

'torpedo' changed from Latin 'torpidō' → medieval Romance forms (e.g. Italian/Spanish 'torpedo', originally referring to the electric ray fish) → English 'torpedo' (applied to explosive underwater devices in the 19th century). The compound 'torpedo-proof' developed in English to describe ships or structures made resistant to torpedoes.

Initially 'torpedo' referred to a fish that numbs (and then to the electric ray); later it came to denote an underwater explosive device. 'Proof' has long meant 'resistant to' (as in 'bullet-proof'). Together, the compound evolved to mean 'resistant to torpedoes' in a naval/military sense.