A noun and a verb. Unless you are a deep sea diver or into wreck salvage you probably won’t have much use for fathom as a noun. It represents a measurement of depth in the ocean equivalent to 1.8m or 6 feet.
The submarine descended to 1050 fathoms before locating the remains of the jet plane.
The verb ‘to fathom’ is in more common use particularly in the negative. It means to be able to understand someone or their motives and it implies having to think deeply about the issue. That the answer is not clear or on the surface. When you are trying to get to the bottom of something you are often fathoming out people’s behaviour.
I can’t fathom why Hilda would throw away a well-paid job and a loving partner to join a charity expedition to save monkeys in Sumatra. Unpaid, stinking hot and miles from anywhere!
Can anyone fathom the reason why the President appointed a ´reformed´criminal as Minister of Police?
Fathom can be used with a preposition – out or on its own as in the example of Hilda above.
It is apparently derived from an old Frisian word meaning outstretched arms, from which the measurement was taken.