A drain is read as release — what is let go or washed away. A blocked drain traditionally stands for something unexpressed building up; things lost down it, for a fear of losing what matters.
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.