心眼 literally means heart holes. There’s an old folk belief (probably based on bad dissection attempts) that having more holes in your heart allows for more complex thoughts.

心眼多 (lots of heart holes) describes someone who is always scheming and thinking several steps ahead of other people. For example, if someone outsmarts you by considering things you didn’t think of, you might say “Damn, 这人心眼真多(this person has so many holes in their heart)”

缺心眼 (lack of heart holes) describes someone who is a simpleton and doesn’t think sufficiently about the things they should be thinking about. It basically means being stupid, although not due to factors such as overconfidence or ignorance, but simply due to not thinking things through and not feeling the need to do so.

If someone falls for a scam, a common thing to say is “下次多长个心眼吧 (Next time you need to grow another hole in your heart)”

Note that none of these are correlated with actual intelligence. It just describes how much you take all the factors into consideration and how willing you are to think things through.

Edit: Just thought of another phrase, 小心眼 (small heart holes) which means narrow-minded and/or stingy.