To fathom something means to understand it deeply, especially when it's complicated or mysterious. When you finally fathom how a magic trick works, you've figured out its secret. When a scientist fathoms a puzzle about how the universe works, she's grasped something that seemed impossible to understand.

The word often appears with “can't” or “couldn't,” describing things that are hard to wrap your mind around. You might say “I can't fathom how anyone could be cruel to animals” or “She couldn't fathom why her friend was suddenly ignoring her.” This usage suggests the idea is so strange or difficult that understanding it feels almost impossible.

A fathom is also a unit of measurement equal to six feet, used mainly for measuring the depth of water. Sailors have used fathoms for centuries to determine if the water is deep enough for their ships. This nautical connection makes sense: when you fathom something, you're measuring its depth, exploring how deep it goes. Just as sailors drop weighted ropes to fathom the ocean floor, when you fathom an idea, you're diving deep to reach the bottom of it.