You know, like who, what, where, when, why, which, and how. Maybe we’ll get an explanation as to why how is the only one not beginning with W. Why don’t we change that?
Who comes from the Old English hwa, which could also mean someone or anyone as well as who. It’s from the Proto Germanic hwas and earlier, the Proto Indo European kwo-, which was the source of a lot of interrogative pronouns, as we’re about to see. No explanation as to why it switched from K to H, but it does seem like the H to W thing is just because the former has softened over the years. And whom is from the same place, just via hwam, which is another version of hwa.
What is from the Old English hwaet, where it could mean what but also who, something...and hark. It’s from the Proto Germanic hwat, which you may recognize as what with the first two letters switched, and the Proto Endo European kwod, which is a form of kwos. Another form for who.
Why comes from the Old English (again) hwi, which was a form of hwaet called the instrumental case. Instrumental is an old grammar form that appeared in Old English (Russian actually still has it) that indicates indirect receivers of action, objects of prepositions, or that a thing is being used. Basically why comes from a form of what that isn’t used anymore and as we all know it comes from the word for who. Although Proto Indo European also had a version of why, kwi, again, another version of kwo.
Okay, you can probably guess at least some of this one. Where comes from the Old English hwaer, which means where. No surprises here. It’s from the Proto Germanic hwar, which is from, all together now, kwo. Are you beginning to see a pattern?
I probably don’t even need to look this one up to guess, but here we go. When is from the Old English whaenne, which means when as a direct question. It’s from the Proto Germanic hwan-/hwa- which… looks very familiar. Dammit, it’s the same one as before and it’s from kwo-.
I’m no longer expecting anything new. Which was hwilc/hwaelc in Old English, and was actually short for hwi-lic, “of what form”. So yeah. Hwi again. And the lic means body (body/form) and is where like comes from. Hwi-lic comes from the Proto Germanic hwa-lik-, and we all should know by now that hwa/hwi comes kwo-.
How comes from the Old English hu, just how. Before that, it’s the Proto Germanic hwo and of course Proto Indo European kwo. No clue as to why this one stuck with H while none of the other ones did. Just weird I guess.
tl;dr: All question words go back to kwo-. It is the one true question.
Tony Jebson’s page on the Origins of Old English