babble → backtalk baby talk (sounds or words of children learning to talk): I was talking baby talk to my little one when he arrived.
backbiting (unpleasant and unkind talk about someone): You should always refrain your tongue from backbiting.
backbone (the most important or strongest part of something, providing support for everything else):
backbreaking (physically hard and very tiring): It was backbreaking task to clean snow from the roof.
backtalk (rude and lack of respect replies to someone in authority): I want none of your backtalk! Just do as you’re told soon.