fathom

noun

• a unit of length equal to six feet (approximately 1.8 m), chiefly used in reference to the depth of water:sonar says that we’re in eighteen fathoms

verb

• 1 [usually with negative] understand (a difficult problem or an enigmatic person) after much thought:he could scarcely fathom the idea that people actually lived in Las Vegas [with clause]:he couldn’t fathom why she was being so anxious

• 2measure the depth of (water):an attempt to fathom the ocean