TFU, an abbreviation which stands for "The Fuckest Uppest", is generally used to describe something which is dangerous due to a capacity for violent unpredictability. It suggests something which is "the most fucked-up" in the sense of being the most unstable, irrational, erratic, easily provoked or insane with anger, and therefore something not to be messed with.
The term first came into popular usage in the BBC comedy programme 'Spaced' (Simon Pegg's character paints the letters on his battle robot in order to make it more intimidating). Apparently TFU is the graffiti signature of one of the members of the Spaced crew.
ED-409 from Robocop is TFU. Compared to him the Terminator is like a special needs child with a silly haircut.