A cofferdam is a watertight chamber allowing construction workers to carry out building work underwater.
Why am I telling you this? Well, today is the day for the letter C, and today I fell over (the two are not connected, incidentally). I was looking for my car (we have to park in the street, and there are a lot of streets and a lot of cars, and I often forget where I've put mine). Having discovered that it wasn't where I thought it was, I was hurrying off in the other direction (bad idea), and I tripped. I had a few bumps and bruises, nothing serious, but being of a certain age (I'v never discovered quite what "a certain age" is, but I have a feeling that it's almost certainly me) I felt a bit shaken. So I cheated, found "cofferdam" in the dictionary, and decided to use that.
To anyone reading, this, I know it's boring. On the other had, you may well have learnt a new word.
PS I never did find the car. (Which, now I come to think of it, also begins with C.)