interjection 1.(used to call or answer someone, or to incite dogs in hunting.)
verb (used without object) 4.to call with a loud voice; shout; cry, as after hunting dogs.
verb (used with object) 5.to incite or chase (something) with shouts and cries of “hallo!”
6.to cry “hallo” to (someone).
Origin:
1560–70; variant of hollo, itself variant of earlier holla < Middle French hola, equivalent to ho ahoy + la there