
The term, Pain-body was coined (as far as I know) by Eckhart Tolle in his first book, The Power of Now: A Guide to Spiritual Enlightenment
. He develops the idea much more fully in his second book, A New Earth: Awakening to Your Life's Purpose.
The pain-body is a complex of built up thought patterns and emotions that results from unprocessed or unacknowledged pain experienced in the past. (Tolle goes so far as to hypothesize that we can be born with a certain amount of pain, but we need not agree with this view for the concept to be of service to us.) It lies dormant for varying periods of time, depending on the person, and is triggered by certain stimuli.