This stands for something that happened a few years ago, before the start of the series. Curtis had just smoked his first jay with Todd. Later in shop class, Todd accidentally cut off Curtis' arm with a saw, probably due to both of them being stoned.
Ever since the accident, shop class has been closed off.
Apparently, both Todd and Curtis both repressed the memory of the accident, and simultaneously remembered it when Curtis had used the book to get a new (demonic) arm in episode 1x13.