Few can deny saying it at some point in their childhood. Is there truth in it? Not really. Words really can and do hurt. Most of us can let some silly things roll off like water on a duck's back, but negativity eats away at our emotional armor and those things get through and penetrate deep into a person's well-being.
If you have children and are teaching them that words mean nothing, I heartily disagree with you. As children, adolescents, and as adults, words hurt. You can't erase them. You can't unsay them. When it comes to hurtful words, we seldom forget them either.
When you make a sideways comment to your spouse about someone at the gas station or grocery store, your children are listening. They are learning to judge people. Be mindful of what you say in front of children, your own or others, and be mindful of what you say even when they are not around.
The girl you call fat may already be struggling with her weight and have self esteem issues. The person you say makes bad decisions make have had some very tough ones to make and you will almost certainly not know the circumstances that surrounded them. When you judge someone for the path they have taken, or where they have arrived at, you are doing so without an inkling of the road they had to traverse to get that far and survive. Even shallow comments can cut deep.
Now, go out there and do it!