An undershirt stands for the plain, unpolished self beneath the public layer. Tradition reads it as honesty and simplicity, or as being seen before you are ready.
Does the traditional reading fit your dream, or did it mean something else to you?
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.