Getting CoD blocked is when you're playing Call of Duty (online or not) and you see a stream of enemies run by that you can easily kill all of them. However, when you pick up your gun to kill them, you get shot in the back (or in the worst case scenario, stabbe in the back)and die before you're able to finish killing them all, or even killing one.
Bren: Check it out guys. There's three of 'em right there.
Vic: Kill them all.
*Bren tries to shoot, but instead sees a replay of him getting stabbed by some random dude. In other words... He gets CoD blocked*