I suppose the origin of the phrase we’ll be looking at today (but not the one in the title!) is fairly obvious, but I don’t think it had really occurred to me till I came across it being used in its literal sense. I watched a documentary, the subject of which I can no longer remember, which featured a very deep well, and someone said it was so deep he ‘couldn’t fathom it’ and I felt sure he wasn’t just using the term loosely but more literally and that it goes back to our old favourite subject, the glorious days of sail.
As many of you will know (that’s if if there are many of you) a fathom is a measurement used at sea and is around six feet in length. To see how much water they’d got beneath them, sailors took soundings with rope that had knots or markers for every fathom – so it seems logical that if they were in deep water a sailor might say ‘I can’t fathom it’, ie can’t get to the bottom of it. Hence the saying that has grown to be used in a more general sense. I also wonder whether the term ‘soundings’ comes from the bodies of water are called ‘sounds’, eg Plymouth Sound. I believe they were often used by larger ships as anchorages – that is, places where you would need to ascertain the depth to make sure you weren’t in danger of running aground.