Merriam-Webster's Collegiate® Dictionary, 11th Edition
FATHOM
I. noun
Etymology: Middle English fadme, from Old English fæthm outstretched arms, length of the outstretched arms; akin to Old Norse fathmr fathom, Latin patēre to be open, pandere to spread out, Greek petannynai Date: before 12th century a unit of length equal to six feet (1.83 meters) used especially for measuring the depth of water, comprehension ,
II. verb
Date: 1607
intransitive verb probe , to take soundings, transitive verb to measure by a sounding line, to penetrate and come to understand , fathomable adjective