Understanding Section 84 of IPC - The 'Idiot' Defence, Seriously?

So, we all know Section 84 of the Indian Penal Code is about insane persons. But have you ever really understood what 'unsound mind' actually means? Our textbooks explain it like a blanket statement, but I've always found it vague. My professor once made a joke that it's like the IPC's way of saying 'kuch toh baitha hai' (something is amiss) in your mind, but how do we quantify that? Is it about being mentally ill, intellectually challenged, or simply not having a 'sound mind'? What's the real-life example that breaks it down? I've heard of cases where someone has committed a crime and their defence is 'I was under the influence of a divine being', but is that really the same as insane? Please help me out, folks!