An exclamation used to call attention: same as hello, hullo, now more common colloquially, and as halloo, which differs more in pronunciation and use. See hello and halloo.
To call or shout to; incite with cries.
Word Usage
"The first tentative explanation of the origin of hallo connected the English word with French au loup, au loup, the cry heard in the chase for setting dogs on the wolf."