Today, my three year old son was doing something he wasn't normally supposed to do but we were letting him get away with it (wearing a backpack to the dinner table). He pointed out that he wasn't supposed to wear it and my wife replied that "today, we're just going to let that slide." Since he's young, the expression (obviously) didn't make sense to him. We explained that it meant that we were going to excuse the infraction today because it wasn't something that was terribly important. He then asked why "let it slide" means that. My wife and I didn't have an answer.
So, I'm here. Where does the expression "let it slide" come from? Why does it mean what it means? And so forth?