What role does もう play in this sentence?

Question

頭痛に頭を掻き毟り{むしり}つつ、震えるもう片方の手が宙を泳いだ。

Does もう modify 泳いだ here, as in "already moved (lit. swam through air/space)"? So that "while X (tearing hair with one hand due to headache), the trembling other hand already moved"?

Answer 1

The もう here is the same as the one in もう一つ "another one".

In your example sentence, [he] is scratching off his head with one arm. The other, shaking arm [was trying to swim in the air or whatever].

So, もう片方の手 means "the other arm" (as opposed to "the one arm").

This もう can be used with arms, items, people, anything. Like

二人の男の人を見た。一人は帽子をかぶって、もう一人は杖をついていた。
I saw two men. One wearing a hat, the other one with a walking stick.