My Mom usually uttered this phrase when we were in the company of other children and began to act silly. Doing unnecessary things just for the sake of doing them, or because the other children were. One look from Mom and the sound of the familiar phrase and we were back to our well behaved 3, 5, and 8 year old selves. By the time each of us hit nine years old the fact that 'we didn't do silly' was second nature to us.
As a fifteen year old I have once again run into the problem of silly, not with kindergartners this time, but with teenagers. Teenagers waste so much time 'doing silly' when they could accomplish so much more by 'doing serious with a dash of fun.' Silly can be anything from making irrelevant jokes to acting like ten year olds in any capacity. I am sure you all have seen plenty of silly from our generation, you know exactly what I am talking about.
Be different, do a hard thing, and let it be known that 'you don't do silly.' Spend these precious years wisely, seriously, accomplishing great things for God. Maybe your seriousness in furthering the Kingdom of God will rub off onto the rest of our generation. Oh, and don't forget to have fun. ;)