Fathom/ˈfæðəm/ Definition 1 of 3Pro Tip 1/3Emotional Weight When you can't fathom something, it often involves shock or surprise. After the twist ending, I couldn't fathom how I'd missed all the clues. Pro Tip 2/3Figuring It Out Used when trying to understand something complex or not immediately obvious. He took hours trying to fathom the equation. Pro Tip 3/3Difficulty Expressing Often used to describe concepts that are hard to put into words. She found the philosophy lectures hard to fathom.