The Latin word for being tired or numb was torpere. From this verb they got the adjective torpid, which is still with us today. And that would be the end of the story were it not for electrical fish.
That there are electric eels is commonly known (and therein, if anybody would listen to me, lies the solution to the world's energy problems. All electrical devices from mobile phones to aeroplanes would have a small fish tank attached and... but I digress). There are kinds of ray that can produce electricity, in fact they can produce 220 volts of the stuff, which is quite enough to knock you out, and therefore make you torpid.
In English they were once called numb-fish or cramp-fish, but the educated Latin name is torpediniformes, with the major family being the torpedoes. As Lawrens Andrewe put it in his snappily titled The noble lyfe & nature of man, Of bestes, serpentys, fowles & fisshes y be moste knowen (1520):
Torpido is a fisshe, but who-so handeleth hym shalbe lame & defe of lymmes that he shall fele no thyng.
Marlowe refers to a chap as a "vile torpedo". There was an eighteenth century dandy called Beau Nash who was awfully witty but had trouble writing well. "He used to call a pen his torpedo for whenever he grasped it, it numbed all his faculties." This is a shame as Nash was very charming. When he died his wife went to live in a hollow tree near Warminster.
Anyway, in 1776 the Americans were revolting. The British Navy sailed to New York, but so revolting were the Americans that the Brits decided to stay in the channel and blockade the harbour. The Americans did not like this and there was a fellow called Bushnell who invented a submarine with which to attack the blockading British boats in the most unsporting manner.
Bushnell couldn't decide what to call his submarine: he seems to have been in two minds between the American Turtle and the Torpedo. In shape it resembled both.
It should be noted at this point that the OED, Wikipedia and the Encyclopedia Britannica are all wrong. I checked. There is a lovely contemporary account of Bushnell's machine to be found here and here. The idea of the submarine, called the Torpedo, was that it had a "magazine or powder" attached to it that it would screw to the hull of the British flagship. The revolting Americans were foiled by the ship's hull, which was copper-bottomed.
The torpedo trying to attach a mine
But the Americans were not to be deterred. Another inventor called Fulton took up where Bushnell left off (Bushnell for some reason ran away to the South and took on a new identity). It was Fulton who decided to call the explosive device a torpedo. He also decided to change it a bit. Rather than the submarine getting right up to the ship it didn't like, it would instead fire a harpoon at it. The explosive device, now called a torpedo, would be attached to the harpoon by a rope and contain within it a timer. So the submarine would pop up, harpoon the ship, and disappear before the charge went off. Full account from 1809 here.
Fulton's torpedoes didn't work either. Decades passed during which the torpedo was fitted with a motor and other such improvements, but nothing was sunk with a vile torpedo until 1878 when a Russian ship sank an Ottoman one.
And that, dear underwater reader, is how tired and numb came to mean fast and explosive.
The Inky Fool in the bath