Hallo

wiktionary

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From Middle English halou, halow, halloo(interjection used to call attention), representing Old English hēlā, ǣlā, ēalā(“O!, alas!, oh!, lo!”), equivalent to hey +‎ lo.

etymonline

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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.