A slip is the thin layer between skin and dress, so tradition reads it as something half-hidden. It can stand for a delicate feeling, or a truth just beneath the polished surface.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.