The Boss: That's all there is to it.
Slevin: Is that all there is to it?
Mr. Goodkat: Yup... That's all there is to it.
Lindsey: Thanks for the sugar, sugar.
Lindsey: I was just thinking that if you're still alive when I get back from work tonight... maybe, I don't know, we could go out to dinner or something?
Lindsey: We are dealing with a bona fide case of mistaken identity here.
Slevin: Yeah.
Lindsey: Things like that aren't supposed to be real. It's like amnesia.
Lindsey: Not withstanding, here you are and Nick's nowhere to be found, so... I'd say you're fucked.
Slevin: Fucked.
Lindsey: Shouldn't you be a little more worried about all this?
Slevin: I have ataraxia.
Lindsey: Ataraxia?
Slevin: It's a condition characterized by freedom from worry or any other preoccupation, really.
Slevin: Ok, I'm under the impression that you're under the impression that I owe you 96,000 dollars.
The Boss: No, you owe Slim Hopkins 96,000 dollars. You owe Slim, Slim owes me... You owe me.
Slevin: I have ataraxia.
Lindsey: Ataraxia?
Slevin: It's a condition characterized by freedom from worry or any other pre-occupation really.
Answer: I've considered 2 possibilities. One being the answer given about his cover, but I think he may have actually had it. When Goodcat is pointing a gun in his face as a child, he doesn't look scared in the least. The death of his parents "killed him" giving him the condition. Later proof is when he tells the Boss "You can only kill me once."