British psychoanalyst Christopher Bollas used this phrase to describe the feeling of something we know at some level but cannot quite explain. A visceral, non-verbal  memory of our early environment that shapes who we are from our earliest experiences of the object, be it mother, father, or self.
This brings to mind the different types of memory we can access. When we think of memory we tend to think of the conscious recall of information, our explicit memory.  Explicit memory describes this system of conscious memory; a system that allows us to learn how to do specific things and recall specific events. However, as well as explicit memory we have implicit memory from a time when our brains were not developed enough to process memory stretching back to before birth. Implicit memory exists in the unconscious patterns that are mostly inaccessible to conscious awareness. Our implicit memories are kept out of our conscious awareness yet they wordlessly seem to creep into our conscious experience and often impact on our lives.
The novelist Barbara Kingsolver wrote ‘it’s surprising how much memory is built around things that go unnoticed at the time’. In therapy we have the opportunity to expand our conscious awareness and integrate both conscious and unconscious memories. Recognising the existence of such memories and the ‘unthought known’ shows us that often we do not need to go back into the past in therapy as the past is already there with us in the room.