FIMming (FIM stands for “Fuck I Missed”) is a training technique designed to increase the output of an individual cannabis plant. It works by cutting off the shoot apical meristem about 2/3rds of the way down to the next node. The cutting can be done with scissors or pinching with the fingers. The result is that four new shoot apical meristems will form, instead of one, or two by topping. Once the process is complete, and the new meristems have had time to form new nodes, the process can be repeated.
As to why it’s named that way… well, that’s anybody’s guess.