Expressed as a decimal, it is a metric of the likelihood that you will finally say, "fuck it" and give up or quit. Most often used in work situations. 0.1 is considered mild frustration, 0.5 annoyance, 0.8+ violent rage/physical pain. 1.0 is the instant you quit, 2.0 is going postal.
During the company meeting when they said we were all getting pay cuts I was at .98 fuck it factor, but now that I'm at the bar I've mellowed to a .86.