Fathom has a few meanings, the most well-known of which is probably as a unit of measurement for the depth of water, one fathom being equal to six feet (1.83 metres for my metric friends). ‘Fathom’ also has a figurative meaning which is when you use it to describe understanding or comprehending something – ‘despite her best efforts, she couldn’t fathom what the hell was going on’.
Fathom comes from an Old English word, fæthm, which means ‘outstretched arms’ or ‘embracing arms’. In fact, ‘fathom’ was once used as a verb to mean ‘embrace’ – so you could say you were going to fathom someone if you were going to hug them (although that does sound vaguely threatening). In the 1600s, ‘fathom’ ran away to sea, and the verb came to mean ‘to measure with a sounding line’ – a sounding line being a bit of rope with a weight on the end used to measure the depth of water. The measurements then became known as fathoms, because sailors used their outstretched arms to measure the length of these ropes or cables (which makes much more sense that what I was imagining – lots of sailors trying to measure the depth of water by getting in the sea and stretching their arms out). Eventually this measure was standardised as six feet.
At the same time as all this was happening on the ocean waves, landlubbers started using ‘fathom’ to mean ‘probe’ or ‘investigate’. That’s because it was all about getting to the bottom of something – just like those sounding lines.