backbrain

The repository of knowledge one retains but doesn't realize one has until such knowledge is necessary in a given situation (e.g. recalling the name/face of an old acquaintance/friend/girlfriend)

"I was walking to work and some guy asked me where I went to school; I thought he was a creep until my backbrain kicked in and I realized we lived on the same floor freshman year."

"I couldn't figure out why she slapped me until my backbrain kicked in and I remembered that I slept with her best friend a few years back."