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).
How is it that, as in How come you're not attending the conference? Sometimes
how come follows a statement and asks the question “why” or “in what way,” as in You're not going? How come? The related phrase
how so? functions the same way, as in You say she's changed her mind—how so?How come is short for how did it come about that and dates from the mid-1800s; how so, short for
how is it so or how is it that
, dates from about 1300.