身上 is a location noun meaning 'on (or in) one's body', but also extends idiomatically to mean 'about one's person' or 'at hand' — something you physically carry or wear. A less common usage indicates 'in someone' (e.g., a quality or disease), roughly 'among' in the sense of 'existing in a person'. The primary frame is physical location on the body, followed by the metaphorical sense of possession or presence.