Don't listen to these fools--they don't know what they're talking about.
When people are talking about MK's, it is an abbreviation for "Mark." Mark is generally a pretty cool guy, as long as you keep him away from the hard liquor, in which case he becomes entirely too unruly. When used in conjunction with numbers, it tells you the current level of Mark's inebriation. Hence, MKI is used to denote that Mark just woke up and hasn't hit the juice yet, MKII happens when Mark has had a few drinks and is starting to become a bit tipsy and may or may not have his pants on around his head. MKIII is to be avoided at all costs, and happens when Mark's temulency has gotten completely out of hand. At this point, Mark is no longer a pretty cool guy, but a raging maniac, bent on ruining your party. He's like that guy who's flailing his arms around, shouting at everyone that he's not drunk and that it's not up to you to tell him if he's drunk or not, that's something only he can decide. He might start throwing inanimate objects around, but will inevitably start throwing small animals and children if such objects are not within his immediate grasping range. By this point, the party will be completely ruined, everyone will be pointing fingers, asking who invited that guy, and the cops will likely be on their way.
I really hope this clarified a few things for you.