(n.) Your "deepest, darkest" secrets, stowed away in a tiny pocket within your mind. When asked to share a secret, we generally skim over these because they are either too dark to be revealed to others, or are locked within our subconscious - because your conscious mind could not handle the vulgarity of the act(s) - and sometimes only in our slumber are we ever aware of these. Pocket secrets are shared on the rarest occasions, if ever.
Barbie: Tell me a secret
Ken: I don't have any secrets, I'm an open book. (Except for my pocket secret about that time I got drunk and slept with Ken Senior and Malibu Barbie! I wouldn't tell anyone that!)