ETYM Old Eng. whi, why, AS. hwî, hwi, instrumental case of hwâ, hwaet; akin to Icel. hvî why, Dan. and Swed. hvi.
Interrogative adverb asking for a cause or motive; for what reason; wherefore.
Question word; what is the reason; ('how come' is a nonstandard variant as in); SYN. how come.