Origin and history of ahoy

greeting between persons meeting, 1848, the early references are to the U.S. western frontier (where hallo, the house was said to be the usual greeting upon approaching a habitation).*

It is an alteration of hallo, itself an alteration of holla, hollo, a shout to attract attention, which seems to go back at least to late 14c. (compare Middle English verb halouen "to shout in the chase," hallouing). OED (1989) cites Old High German hala, hola, emphatic imperative of halon, holon "to fetch," "used especially in hailing a ferryman." Fowler, an Englishman, in the 1920s listed variants halloo, hallo, halloa, halloo, hello, hillo, hilloa, holla, holler, hollo, holloa, hollow, hullo, and writes, "The multiplicity of forms is bewildering ...."

Its rise to popularity as a greeting (1880s) coincides with the spread of the telephone, where it won out as the word said in answering, over Alexander Graham Bell's suggestion, ahoy.

Hello, formerly an Americanism, is now nearly as common as hullo in Britain (Say who you are; do not just say 'hello' is the warning given in our telephone directories) and the Englishman cannot be expected to give up the right to say hello if he likes it better than his native hullo. [H.W. Fowler, "A Dictionary of Modern English Usage," 1926]

Central telephone exchange operators were known as hello-girls by 1880.

An effort is being made to establish telephonic communication across the Atlantic, through the agency of the Atlantic cable. Fancy Garfield at the American end with the transmitter at his ear, and a French hello girl telling him that President Grevy wants to congratulate him on his election. [Canton (Ohio) Advocate, Oct. 14, 1880] 

*

    "Hallo, the house!" said a well-dressed gentleman, on horseback, as he approached a log hut in the centre of a large pine forest. ["Sophia, Or the Girl of the Pine Woods," in Fall River (Mass.) Monthly, Aug. 26, 1826]

     "Are Mr. and Mrs. Ashley in the house? I would see them at the door for one moment."

     "Halloo! halloo the house! Judge Ashley and lady, halloo!"

["May Martin" ("A Green Mountain Tale"), D.F. Thompson, 1837]

     "HAL-LOO!" A dog barked. "Hal-loo the house there!" in a very loud voice, for the house was some distance from the street, in a beautiful square of ground, decorated with evergreens, and the preacher was anxious to find a resting-place. [Edward Eggleston, "The Circuit Rider," 1856, set on Virginia frontier c. 1800]

1962, vulgar or working class pronunciation of hoy a call or shout to attract attention (compare ahoy).