fathomless

Fathomless is an adjective used to describe something so deep that it cannot be measured with a fathom, or so difficult to understand that its extent cannot be known. The term derives from fathom, a nautical unit equal to six feet, combined with the suffix -less. The exact origin of the unit fathom is historical and tied to measurements used by sailors, often linked to the span of outstretched arms.

In literal usage, fathomless describes vast bodies of water or depths that seem immeasurable, such as fathomless abysses or chasms. In figurative usage, it characterizes things that resist comprehension or knowledge: a fathomless mystery, a fathomless grief. The word is common in literary and descriptive prose, particularly in maritime or exploratory contexts, to convey vastness, incomprehensibility, or inexhaustible quality.

Related terms include unfathomable, which is often used to emphasize incomprehensibility rather than physical depth. While fathomless can imply extreme depth, unfathomable tends to focus more on the cognitive or conceptual impossibility of fully understanding something.

See also: fathom; unfathomable.