Ahoy is a signal word used to call a ship or boat. It is derived from the Middle English cry, 'Hoy!'. The word fell out of use at one time but was revived when sailing became a popular sport. 'Ahoy' can also be used as a greeting, a warning, or a farewell. Alexander Graham Bell initially suggested that the standard greeting when answering a telephone should be 'ahoy', but instead 'hello' (suggested by Thomas Edison) was adopted.

Source: Wikipedia