2 Samuel 20:3

But went not in unto them He looked upon them as become impure to him, having been defiled by his son. They were shut up to the day of their death, living in widowhood Being royal wives, it was not proper they should be married to any one else, and therefore David did not give them a bill of divorce, but shut them up close, that no man might have converse with them. And indeed it would not have been prudent to have let them be so much as seen abroad, as that would have renewed the remembrance of Absalom's crime.