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Unit of length indicating the depth of the sea

Symbol: f. / Base data: length. SI base unit, Imperial units.
1f. = 6 feet = 1.8288 m

Word originating from the outstretched arms of pirates

In the United Kingdom, “fathom” is used to measure the depth of the sea. This is the span of a man’s outstretched arms and is the same as the ancient Japanese unit of “hiro”. But, why is the span of outstretched arms used to measure the depth of the sea? The word apparently originates from the measurement of outstretched arms as started by the Vikings who sailed the seas and pillaged in the middles ages.