Your back is most commonly about your past as like your past it is always behind you.
Washing your back whether deliberately or inadvertently is about the need to cleanse some baggage you are carrying from the past. For example, a window cleaner spilling soapy water on your back is a request to cleanse issues from the past.
Looking over your back or looking out a back window means that you are focused on the past. Something in your past has you looking back at it all the time. You may not be consciously aware of it but whatever you are doing today is controlled by something from the past.
If someone has their back to you it means the person they symbolize in the dream turned their back on you. At least that’s your view of it, but when it comes to your life you can only see it from your point of view. If the person in the dream is a woman, this is most likely going to be your mother. If the person in the dream is a man this is most likely going to be your father.
