Depends on context: if the floor is slippery then 小心=be careful. If it's a more urgent situation then 小心= watch out.
I agree that that would be a better default translation, even if "watch out" might be appropriate in certain situations.
I’d like to know too.
I know that Egyptian and subsequent Greek ideas involved the heart as the seat of intelligence ... so it would make sense that if you want to be aware of, or pay attention to, something you’d devote a little bit of your mind (located in your heart) to that thing - so ‘(focus a) small (bit of your) heart’ would make sense.
But I have no idea what Classical Chinese thinking was about where the seat of the mind would be.
That's interesting. Can someone explain how "small heart" came to mean "be careful"?