Origin and history of how

Old English hu "how," from Proto-Germanic *hwo (source also of Old Saxon hwo, Old Frisian, Middle Dutch hu, Dutch hoe, German wie, Gothic hvaiwa "how"), an adverbial form from PIE root *kwo-, stem of relative and interrogative pronouns. Practically a doublet of why, differentiated in form and use.

How come? for "why?" is recorded by 1848 [Bartlett], probably from earlier fuller expressions, e.g. "... how come we to think one thing right in our childhood, and another in our manhood ; how come we to sacrifice one set of opinions to another ...?" [New Monthly Magazine, 1829]; "My father's horses? What! Are they dead. How come they die?" [The Newbernian, Feb. 16, 1838]

Emphatic phrase and how! is recorded from 1865. The formulation was common in book and article titles ("The National Debt, and How to Pay It"), but Pennsylvania writer Bayard Taylor, in whom it is first recorded, seems to have regarded it as a German or German-American expression.

Native American greeting, Siouxan (Dakota hao, Omaha hau), first recorded 1817 in English. But according to OED, the same word was noted early 17c. by French missionary Jean de Brebeuf among Hurons as an expression of approval (1636).