A person says "bite your tongue" to someone else when they feel the person has something they shouldn't. Bite your tongue means to quit talking -- You've said something you shouldn't have said.
I'm pretty sure it means to be careful about what you say and think before you speak (which by the way isn't something I do very often) and if it's unnecessary to hold back.
In your dreams means that it is almost impossible.Bite you tongue means that you should really be quite or else you will get yourself in to a heap of trouble!!