hallo interj.

1580s as a command to get attention, in which use it belongs in the group with hello, hallo. From 1520s as a command to "stop, cease," from French holà (15c.), which "Century Dictionary" analyzes as ho! + la "there." As an urban slang form of holler (v.) "greet, shout out to," it was in use by 2003.

greeting between persons meeting, 1848, early references are to the U.S. western frontier (where hello, 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 cites Old High German hala, hola, emphatic imperative of halon, holon "to fetch," "used especially in hailing a ferryman." Fowler in the 1920s listed 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. Central telephone exchange operators were known as hello-girls (1889).

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]

see hallo.