The first one must refer to the adj "stupid", meaning that at the time of speaking it's the only thing you can think of about that person. (or the best thing you can remember)
And about the second one, it means before putting that item in the boot you can separate or devide it into those component parts. (the item could be something consisting of some components such as a rack.)
I hope someone would correct me if there's anything wrong.
*not a teacher*
Student or Learner