HELLO, is it? Hollo! Hallo! Halloo! is what your ancient Old High German/Old French predecessor did shout, bellow, exclaim, roar. A loud cry to call attention: quarry spotted, trespasser warned, ferryman hailed, objection exclaimed, greeting chorused, champion exhorted! These days, it is tamed to the decorus emollient of everyday encounter…but still we sometimes holler.
Etymological illuminations brought to you by Jane
The Other Day at Portrait... 
“tamed to the decorus emollient of everyday encounter…” Oh, Jane, how I miss you and all of your emollient hellos!
(love this post)
Hey Jane,
I enjoyed reading your etymological, alliterical (is there an adjective for alliteration?) explanations. I leave you with the modern, street form of the word which appropriately fits the occasion: Holla!