hallo (interj.)

shout to call attention, 1781, earlier hollo, holla (also see hello). "Such forms, being mere syllables to call attention, are freely varied for sonorous effect" [Century Dictionary]. Old English had ea la. Halow as a shipman's cry to incite effort is from mid-15c.; Halloo as a verb, "to pursue with shouts, to shout in the chase," is from late 14c. Compare also harou, cry of distress, late 13c., from French.