The nightcap is read as winding down and closing the day. Whether a cap for sleeping or a last drink, dream books see it as the wish to let go of tension and rest.
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.