Stands for shot around corner. Mostly used in the Xbox 360 game Call of Duty Modern Warfare 2. Because it is very common on multiplayer to be shot and killed after you have "ran around a corner" or some other type of cover. This shows up very well when you watch the kill cam.
SAC means suck.adams.choad... it is a commonly used abrivation. Its somewhat like saying SMD. Or.. in matts word... suck african cock... or suck alotta cream.. but it is again most commonly used as 'Suck Adams Choad'