I can't tell you how many times I've looked up the etymology of hello...but for some reason, I've never shared. Obviously time to remedy that!
So the life of hello began with Old High German's hala, hola. It was an imperative form of halon, holon, which meant "to fetch." It was what people cried out to ferry boat captains to get them to pick one up.
English adopted it as early as the 1400s, using it as an exclamation meant to attract attention, spelling it holla, or hollo. But they didn't stop there--they also came up with a "bewildering" amount of other forms, like:
Hello didn't catch on among the British for most of history. Hullo became the standard there, while in the 1880s, America adopted hello as their standard form. It rocketed to the forefront of American speech with the advent of the telephone. Alexander Graham Bell and his cohorts agreed there should be a standard greeting when answering the phone. Bell wanted "ahoy" (which I think is super fun, so I've been known to answer like that when it's my hubby calling), but "hello" won out.
And now hello is quite common in Britain:
Hello, formerly an Americanism, is now nearly as common as hullo in Britain (Say who you are; do not just say 'hello' is the warning given in our telephone directories) and the Englishman cannot be expected to give up the right to say hello if he likes it better than his native hullo. [H.W. Fowler, "A Dictionary of Modern English Usage," 1926]