fathom

Concise Oxford English Dictionary © 2008 Oxford University Press:
fathom/ˈfaðəm/

▶noun a unit of length equal to six feet (1.8 metres), chiefly used in reference to the depth of water. ▶verb

• 1 [usu. with neg.] understand (something) after much thought.

• 2 measure the depth of.

– derivatives
fathomable adjective,
fathomless adjective.

word history: The original sense of the word fathom, in Old English, was ‘the enclosure formed by the breast and arms’; fathoms were ‘arms extended in an embrace’, or ‘outstretched arms’. From this the word came to represent a unit of measurement, based on the span of the outstretched arms to the tips of the fingers, later standardized to six feet. Other early units of measurement, such as the ell and the cubit, were similarly based on the length of the arm or forearm.