A small child or a teenager who's balls haven't dropped yet. These kids have very high pitched voices, yell alot and agitate everybody wearing a mic. They usualy hear them playing Call of Duty, (not Halo 3 cause every kid on that game have high pitched voices), Gears of War and Nazi Zombies that fall into it's own category(this is where the 'M' in 'M Kids' comes from, the games they play). They are as annoying as kids who don't cover their mouths when they sneeze or cough.
M Kid: I just got the PPSH!
Annoyed kid: Dude I don't give a shit
M Kid: It's such a great gun against zombies!
Annoyed kid: Shut up
M Kid: Ooo I just shot his arms off then I knifed him!
Annoyed kid: Shut Up!
M Kid: I sent you a friend invite to you!
Annoyed kid: I don't give a shit what you did!