A martial art in which the user's limbs move in time as well as space, allowing them to go back in time and punch the opponent repeatedly so that the opponent feels the effects of past blows while standing in front of the apparently motionless Déjà Fu practitioner. Similar tricks can be used to render the opponent's attacks ineffective or otherwise incapacitate him.
It is best described as "the feeling that you have been kicked in the head this way before".
The only known master of Déjà Fu is Lu-Tze. This is one of the reasons that people who do not recognize him are advised to follow Rule One. His mastery of it is whispered about in awe by acolytes and senior, high-level djim ninja monks of time. Even the Dojo Master had to submit to Lu-Tze, as did Lobsang Ludd in his incarnation as Time.
In the construct déjà-vu, literally "already seen" or "already perceived", vu is the past participle of the verb voir, "to see".
However, in the construct déjà-fu, the "fu" part is derived from the past tense of the verb foutre.
Its mildest meaning in French (alongside related terms such as "fou", "foutu") is "mad" or "crazy". Reference Obelix tapping his head and whispering to Asterix Ils sont fous, ces Anglais/Allemandes/Suisses/Espagnols/Languedocoises depending on which foreign country/other part of France with strange local customs they are in).
Please refer to the entries -ing and the link from there to to "love-in-a-canoe coffee", to get the idea of a precise translation of the French verb foutre.
And yes, the verb in French has the full range of meaning of its counterpart in English, including that of "rendered useless" or "damaged beyond repair". (Ref. the American English acronym SNAFU). Which must be the condition of the Yeti-hunters, after Lu-Tze and Lobsang Ludd have finished explaining to them why it is advisable to take heed of Rule One.
On the other hand, it could just be a pun on the martial art kung fu. But that's less fun. Unless it's a neat pun on Kung-Fu and déjà-vu - the most aggressive martial art, and seeing things before they happen. Maybe that's how he does it...
Or perhaps what Pratchett-watchers have come to recognize as a trademark multi-level pun: a seemingly innocent simple one-level gag, which on closer examination goes deeper still?