A fathom is a unit of measure equal to 2 yards or 6 feet, or 1.8288 metres. It was originally defined as the span of a man's outstretched arms, and its name derives from the Old English word faethm, meaning 'bosom'. The unit was first used for land measurement, but is now restricted to nautical uses. After metrication, its use for marine navigation was allowed to continue in the UK and Ireland by directives 80/181/EEC and 89/617/EEC, but this authorisation expired in December 31 1999.
The rationale for the outstretched arm origin, is that to measure depth, sailors used to throw a line with a small weight tied at the end into the water, wait until it hit the bottom, and then measured the length of the line from finger tip to finger tip as it was pulled up again.