For turned based strategy games, it means to keep your units together usually in or near their default positions on the map and use defensive tactics such as attacking only when the enemy is near you, using defend commands or other units to defend each other, etc.
In fighting games, it means to keep defending and block all of your enemies attacks untill there is an opportunity to strike.
Basically, in both cases it's using defense/blocking untill you have a good opportunity to make an action or attack your enemy.
"In Front Mission in the 2nd or so battle against Driscoll's forces, I turtled and didn't move my units then attacked as soon as the enemy came near by"
The act of turning one's backpack inside out. Typically done to a classmate when he/she is zoned out, or just very involved in the lesson that day.
A very funny practical joke, that will be appearing in the 2014 Olympics as Speed Turtling.
Usually done by ninja-like people, who are adept in the art of stealth. If the turtlee happens to catch the turtler in the act, the turtler has to return the contents of the backpack to their original state.
"I don't know man, looks like you got turtled! Haha!"
"What the heck is that??"
"Can't you see your backpack? Looks like that ninja kid in the back of class has done some turtling..."
*struggles to get pencil*
"I can't find my homework, it must have gotten thrown away last time my backpack got tutled"
Mary Madison: What did you do?
Samie: I did the only thing I could do, I began turtling.