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.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.