An exclamative(exclamatory) expression with roots to the Indian "hiyo hiyo" chant. Was used in the Lone Rangers comics and shows. And now decades later was used in music and movies as well. Although the interpretation remains much the same, large numbers of people somehow associate it with greetings(which is false).
It is used unecessarily and unsincerely these days due to most people's ignorance and failure to understand its history and true meaning.
man1:"Everyone is saying Hiyo for no reason and out of the blue."
man2:"Yes...it is used in the movie "Anchorman" and the song "Naive melody" in a completely different way."
man1:"exactly, these kids using it these days must be trying to impress people...although they're failing miserably."