I assume you are measuring the voltage with no load on the adapter, and using an electronic voltmeter (either a digital multimeter or an analogue FET multimeter). The primary load element for the adapter's filter circuit lives in the laptop. (Micro format desktops often use a similar power supply arrangement.) Since the input impedence of the meter is generally in the hundreds of megohms for an electronic voltmeter, it cannot draw enough current from the power supply to filter the output to an approximately constant-voltage DC current. What you are seeing is the raw output of the rectifier (or switching circuit) before it's filtered.
Whether you can measure DC without involving the laptop circuitry depends on where the filter components live. We know that the laptop is going to provide the resistive load; whether the capacitive element lives in the power supply case or the laptop is another question. If it's in the PS case, then you should be able to measure a DC voltage if you shunt a moderate resistance across the power supply (something on the order of 1-4.7KΩ). You should also see the "AC" component reduced to negligible levels (or at least significantly reduced, depending on what resistance the filter is actually expecting). Keep in mind, though, that the laptop power supply is always going to be moderated and governed by the internal power supply circuitry (voltage regulation, charge governing, etc.) in the laptop, so the DC in from the external power supply can be relatively "dirty".
As for the shock hazard, your body will form a resistive load on the power supply, so in the absolute worst case (say your fingers are covered in vinegar and there's room to independently touch both contact areas), you will be an ideal low-resistance load and should see no more than the DC equivalent output of the power supply. It can kill you, but only if you open your chest cavity (or cranium) and apply the voltage directly and locally to the bits you want to fry.
