That is a car battery terminal. That is where you clamp the red or black wires to the car's battery. Everyone who has ever had cause to open the hood of a car and glimpse the battery, knows what a terminal is, even if they don't know they know it.
Here's the thing about battery terminals - they are subject to corrosion. And corroded terminals are a bloody nuisance. They cause your car - which has had its alternator replaced twice in one week - to lose its charge as it sits in your driveway overnight with the engine off, so that when you go out a bit early, intending to run an errand before heading off to work...You find yourself subjected to that increasingly familiar experience of sitting in a car that is so dead, even the dashboard lights quit on you when you attempt to start it.
And Your mechanic is then subjected to the weekly wail of a stranded and very fed-up female.
"What's your car doing?" the laconic and unflappable English voice demands.
"Its not doing anything. Its just sitting there, and won't start."
"All right, Lovely, I'll tell you what you do. Just pop up under the hood, and look for the black wire on the battery terminal and give it a bit of a jiggle."
"Battery terminal" you repeat dutifully, walking out, phone in hand to the car.
"Right. It will be the one on the right - right up there under the fender. Just give it a bit of a jiggle."
You obediently pop up under the hood and jiggle the black wire on the terminal - which you already knew how to find - it being, after all, the only jiggleable black thing on the battery. Then you try it again - upon which the car gives you a wheezy cough before lapsing back into a coma.
"It didn't start." you say.
"Riiiight. Ok, then. Here's what I want you to do. Boil up some water, right? It needs to be boiling...."
"And then you pour it all over the terminal. Got that? Boiling water. All over. Give it a jiggle. And it will start right up and run forever."
"And be really, really careful not to get any of the water that will come pouring off on to your clothes. Because anywhere it splashes.... it will leave a little hole."
"Right. Give me a call back if it doesn't start. That's all it needs - well a new terminal really, but I can't do that from here. My arms aren't long enough. So just pour boiling water all over it and that will be you."
So, after vowing to replace the terminal as soon as possible, you take yourself off to boil water and fetch it back. You pour boiling water all over the terminal, taking care to avoid splashing little holes on to your clothing. You gingerly give the black terminal wire another bit of a jiggle and sit behind the wheel, feeling something like a mad scientist, about to test his newest theory. To your relief, the car does start right up, as though its brush with death never happened. Whether it will run forever remains to be seen, but so far, it has carried through the day. You might not have to hit it with a hammer after all.
And you know a new trick. Fancy that.