Refers to the disorder which manifests itself when you inappropriately make a statement at an inopportune moment, often deemed irrecoverable - thus you have inserted your foot into your mouth.
Typically followed with embarassment & many apologies, or a tacit understanding about your position.
Bob (whispers): "uhhhhhh he's right behind you..."
Bob: "Nice case of FIMS there ace"
FIMing is the act of using FIM. It has become a popular term at San Diego State University where it originated.
Fred: "that might work because you are a bitch"
Tommy: "thanks, but I might get some from it"
Fim: yea, but get someone better first...
JaXHaX: wtf you faggot...come or spec..u choose