O.E. hu, from W.Gmc. *khwo- (cf. O.S. hwo, O.Fris., M.Du. hu, Du. hoe, Ger. wie, Goth. hvaiwa "how"), from common PIE interrogative pronomial stem. However is M.E.; how come? for "why?" is recorded from 1848. And how! emphatic, first recorded 1865, said to be a Ger.-Amer. colloquialism.
Native American greeting, Siouxan (cf. Dakota hao, Omaha hau); first recorded 1817 in Eng, but noted early 17c. by Fr. missionary Jean de Brebeuf among Hurons as an expression of approval (1636).