Is what my 4 year old DS said to me when I asked him to wash his hands. 
He picked up this lovely little phrase from Toy Story, started shouting it at his twin brother - who shouted it back, and it became a bit of an "in thing" for them.
At first I ignored it because I thought they would stop.
Then they started shouting it in public, either in a shop or toilet or anywhere lots of people could hear and judge.
So then I reminded them to use kind words yada yada yada. Or hissed stop it right now, depending on how many looks I got or how fed up I was.
It seems to be a bit of a habit now. They say it to us (me and dp) which they know is wrong, a lot of the time they apologise straight away because it slipped out, or when they have calmed down (if they blurt it out in anger)
What do I do ? 
I didn't expect this back chat and insults malarkey to happen so soon. I thought I had until the teenage years for that.
Is it normal ?