The word "su" is a short form of "subunit" or "subject." It can also be used to refer to a person who is the subordinate or junior to someone else. Full definition
This procedure was then labelled the illusionist ceiling painting of the quadratura (di sotto in su). (thetravelmagazine.net)
From the Merriam - Webster's Collegiate Dictionary: Su - pi - ne: Main Entry: 1supine Pronunciation: * s * - * p * n Function: noun Etymology: Middle English supyn, from Late Latin SU PI NUM, from Latin, neu ter of su pi nu s, adjective Date: 15th century (religion.blogs.cnn.com)