how (adv.) Look up how at
Old English hu "how," from Proto-Germanic *hwo (cognates: Old Saxon hwo, Old Frisian, Middle Dutch hu, Dutch hoe, German wie, Gothic hvaiwa "how"), an adverbial form from the PIE interrogative pronominal root *kwo- (see who). Practically a doublet of why, differentiated in form and use.

How come? for "why?" is recorded from 1848 [Bartlett]. 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.
how (interj.) Look up how at
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).