"What do his legs want?"
Translation:उसके पैरों को क्या चाहिए?
I've never heard of such an idiom. May be the feet are sentient.
(Idle feet movie, anyone?)
ok leaving aside the fact that this sentence is all sorts of weird, why do you need को ?
"को" acts here as a marker of the dative/indirect object ("to, for X"). It's used, among other things, for someone who receives something given ("I give HIM the gift"), or who experiences something ("To ME, there is cold" -> "I am/feel cold"). In Hindi, the normal way of describing wanting is to say that something "is wanted to/for" someone ("To his legs, what is wanted?" -> "What do his legs want?").