FATHOM

v. get to the bottom of; penetrate and understand; measure the depth of

n. unit of length equal to six feet (used for measuring water depth)

For the comics character by Michael Turner see Fathom (comics) A fathom is a unit of length in the Imperial system (and the derived U.S. customary units). The name derives from the Old English word fæðm meaning 'a pair of outstretched arms'. In Middle English it was fathme. Its size varied slightly depending on its base definition, either based on a thousandth of an (Admiralty) nautical mile  or as a multiple of the imperial yard. There are 2 yards (6 feet) in a fathom.
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Noun
1. a linear unit of measurement (equal to 6 feet) for water depth
(synonym) fthm
(hypernym) linear unit
(part-meronym) yard, pace
(derivation) sound
2. (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore
(synonym) fthm
(hypernym) volume unit, capacity unit, capacity measure, cubage unit, cubic measure, cubic content unit, displacement unit, cubature unit
(classification) mining, excavation
Verb
1. come to understand
(synonym) penetrate, bottom
(hypernym) understand
2. measure the depth of (a body of water) with a sounding line
(synonym) sound
(hypernym) quantify, measure
(derivation) fthm

n. fathom, unit of length equal to six feet (used for measuring water depth)

See: Weights And Measures AND Measures