Compulsive Lying is one of Fallout GURPS disadvantages. See: Fallout GURPS.
The opposite of Truthfulness, the Compulsive Lying disadvantage forces the character to lie constantly, for no reason other than the joy of telling the tale. A compulsive liar delights in inventing stories about his deeds, lineage, wealth - whatever seems as though it might impress his audience. Even when exposed as a liar, the character will cling tenaciously to his stories, calling his accuser a liar and a scoundrel.
In order to tell the pure, unvarnished truth, a compulsive liar must roll against IQ-4. A charitable GM might allow a liar to tell a slightly-fractured version of the truth if he only narrowly failed this roll. When a PC liar makes a roll to tell the truth to his fellow party members, he should roll out of sight of the other players. Thus, they can never be sure that they are getting accurate information from their comrade.”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Compulsive Lying was removed from the game