Wikipedia to the rescue
According to the Oxford English Dictionary, hello is an alteration of hallo, hollo, which came from Old High German "halâ, holâ, emphatic imper[ative] of halôn, holôn to fetch, used esp[ecially] in hailing a ferryman." It also connects the development of hello to the influence of an earlier form, holla, whose origin is in the French holà (roughly, 'whoa there!', from French là 'there').
( The Light in Chains don't touch the Pope's boner, Wed 26 Aug 2009, 12:15, Reply)