If something is out of your MERLIN (or Mental Realm Capacity) it means that it is out of your understanding, knowledge or comprehension.
Person 1: Hey do you know how this works?
Person 2: No, It's out of my MERLIN
Person 1: Your what?
Person 2: my MERLIN, Mental Realm Capacity
Person 1: What?
Person 2: you wouldn't get it, it's out of your MERLIN