hallo (interj.) Look up hallo at Dictionary.com
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.