FATHOM (a word common, in various forms, to Scandinavian and Teutonic languages; cf. Danish favn, Dutch vaam and Ger. Faden, and meaning "the arms extended"; the ultimate origin is a root pet, seen in the Gr., to spread), a measure of length, being the distance from the tip of one middle finger to the tip of the other, when the arms are stretched out to their widest extent. This length has been standardized to a measure of 6 ft., and as such is used mainly in soundings as a unit for measuring the depth of the sea. "Fathom" is also used in the measurement of timber, when it is equivalent to 6 ft. sq.; similarly, in mining, a fathom is a portion of ground running the whole thickness of the vein of ore, and is 6 ft. in breadth and thickness. The verb "to fathom," i.e. to sound or measure with a fathom-line, is used figuratively, meaning to go into a subject deeply, to penetrate, or to explore thoroughly.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)