When you're such a pathological liar that every time you say something of importance everyone asks you.."do you swear on the holy bible?" In order to confirm the truth... saying the shortened "bible" just minimizes the length of the asking while not compromising its purpose.
"Man, I fucked that 300 pound chick last night." - Me
"Bible?" - My friends
"Bible!" - Me
"Holy fuck, I thought you were lying about that, you're sick dude" - My friends