Torpedo did not originally mean a submarine weapon. It first referred to an electric ray, a flat fish that can produce a strong electric shock. The name comes from Latin, where torpere means to be numb or stunned.
That older meaning makes sense once you know what the animal does. People used the word for a ray because its shock could leave other animals, or even a person, temporarily numbed.
- Original meaning: a torpedo ray, an electric fish
- Later meaning: a torpedo, an underwater explosive weapon
The weapon sense came later because the idea of a sudden disabling strike fit the older word. Over time, the military meaning became much more common, so many readers now only know torpedo as a naval term.
In short, the word moved from a shocking fish to a shocking weapon. The animal came first, and the weapon borrowed its name.