I found the following dialogue on lang-8:
A: 駄目だ、飲み過ぎて頭が回転しない。 B: 仕事、明日にしたら？
A: Oh no, I drank too much so I can't concentrate on it. B: Why not do your work tomorrow?
So does 頭が回転する mean "able to concentrate" or am I misunderstanding it? In English when ones head spins, it is a euphemism for being drunk or dizzy but I guess in Japanese having ones head revolve quickly (maybe describing the speed of thoughts turning over/ideas coming) equals being quick-witted. Would anyone be kind enough to explain this idiom?