electric shock from plugged in lightning cable

What you are sensing are called "parasitic currents" and they are not harmful, although disconcerting. The case of your Mac is grounded for safety. And there is a ground connection in the tip of the lightning cable, also for safety. However, "ground" is not a fixed condition. When current flows in a ground wire the resistance of the wire, although very small, is not zero (mathematically, E=I*R, or the voltage is the current times the resistance). So there will be a voltage difference between one end of the wire and the other. If you touch both ends you will feel this small voltage difference, especially if your skin is damp.

Now extend this to the cable and laptop. There's no current flow in the lightning cable, so if you touched both ends you wouldn't feel anything. However, there is current flowing through the ground wire in the laptop power cord, so there will be a small voltage between its case and, say, a cold water pipe (which is a standard ground reference). The lightning cable is at the same potential (fancy word for voltage) as that cold water pipe, so you will feel this voltage difference. Your body is very sensitive to voltage measurement, so it can detect the millivolt range difference. But voltage differences are not really harmful on unbroken skin until they get up over 50 volts or so. That's why you can touch both terminals of a 12 volt car battery without feeling anything. As another experiment, touch your tongue to both terminals of a 9 volt battery; it won't be painful, but you will be able to sense the voltage across the terminals.