Which word refers to something difficult to understand or fathom?

The term that refers to something difficult to understand or fathom is "inscrutable." This word is often used to describe a person, situation, or concept that is mysterious, enigmatic, or not easily interpreted. For example, an inscrutable expression might imply that someone's emotions are hidden, making it hard for others to read or understand their thoughts and feelings.

In contrast, the other terms suggest different meanings. "Incongruous" typically relates to something that is out of place or inconsistent with its surroundings. "Insensible" refers to a lack of awareness or the inability to feel or perceive, rather than a lack of understanding. "Intransigent" describes someone unwilling to change their views or disagreeing stubbornly, which does not pertain to understanding or comprehension. Thus, "inscrutable" stands out as the most appropriate choice for the given definition.