A hidden, private part of the body, the armpit is traditionally read as something you keep close and would rather nobody saw. Dreaming of it exposed often mirrors a worry about being seen at your least polished.
Where in your waking life have you met something like this lately?
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.