A disguised 'wh-' word — OE 'hū' lost its /w/ before a rounded vowel while 'what,' 'who,' and 'where' kept theirs.
In what way or manner; by what means; to what extent or degree.
From Old English 'hū' (how, in what way), from Proto-Germanic *hwō, from PIE *kʷō, an instrumental or manner formation on the interrogative stem *kʷó-. Unlike the other 'wh-' words, 'how' does not begin with 'wh-' in modern spelling — it reflects a different phonological development where the Old English 'hw-' became simply 'h-' before the rounded vowel. The Latin cognate is 'quō' (where to, by which way
'How' is actually a 'wh-' word in disguise. Old English spelled it 'hū,' but the Proto-Germanic ancestor was *hwō — the same *hw- cluster found in 'what,' 'who,' 'where,' and 'when.' The /w/ was absorbed by the following rounded vowel, which is why 'how' begins with 'h-' while its siblings begin with 'wh-.' It is etymologically *'whow.'