fath·om 
(făthəm)
(făthəm)n. Abbr. fth. or fm.
A unit of length equal to 6 feet (1.83 meters), used principally in the measurement and specification of marine depths.
tr.v. fath·omed, fath·om·ing, fath·oms
1. To penetrate to the meaning or nature of; comprehend.
2. To determine the depth of; sound.
[Middle English fathme, from Old English fæthm, outstretched arms; see petə- in the Appendix of Indo-European roots.]
fathom·a·ble adj.