Fathom to the Old High German word "fadum" meaning embracing arms or a pair of outstretched arms. In Middle English it was "fathme". The Ancient Greek measure known as the orguia (, "orgyiá", "outstretched") is usually translated as "fathom". By the Byzantine period, this unit came in two forms: a "simple orguia" (, "haplē orguiá") roughly equivalent to the old Greek fathom (6 Byzantine feet, m) and an "imperial" (, "basilikē") or "geometric orguia" (, "geōmetrikē orguiá") that was one-eighth longer (6 feet and a span, m). One fathom is equal to: In the international yard and pound agreement of 1959
Fathom Castle was being built c. 1180-1200, it most probably was a fortified structure occupied by a knight in the service of DeLacy. There is definite mention of Sir Robert Marmion being at a prest of knights before King John in Dublin in 1210. It would appear that this Sir Robert was the knight responsible for Fidun/Fathom Castle. He entered Ireland with Strongbow and other Normans in 1169-1172 as documented, and there is mention of other Marmions, presumably his descendants being 'of Fidun', i.e. Sir Gilbert in 1290 and a William as late as 1305. With the Gaelic resurgence, the 'Fidun' location