Kriff was a vulgar expletive. Kriffing was a derogatory modifier based on the word kriff.
Kas'im used this expletive when he tried to argue with Qordis for restricting his student's actions inside the Korriban Academy and Republic soldiers frequently used this expletive when talking about the war on Ruusan against the Brotherhood of Darkness. Luke Skywalker used the word kriffing to refer to slicers when R2-D2 refused to let him see the contents of a protected memory area, and had gone so far as to damage himself. Luke had forgotten that his young son Ben Skywalker was present, and when Ben asked Luke what kriffing meant, Luke remarked that Ben's mother would not be pleased that Ben overheard Luke using that word. Captain Lon Shevu of the Galactic Alliance Guard used the word often when on raids, as well as Han Solo.