Ahoy is used to get attention or greet. A ship's watch may shout 'land ahoy' or 'ship ahoy' when sightings are made.
It is thought to have originated from Middle English (15th Century) 'Hoy'. Hoy would have been used as a greeting or to get peoples attention particularly on boats. It is a sound that carries well over distances and possibly never had a translation and was always just a sound or call.
Interestingly, 'Hoy' is also a type of barge or fishing vessel used in the 17th and 18th century, so maybe there is a connection here as well.
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