An expression used as a reply (or a sarcastic encouragement) to people who are obviously stretching the truth, exaggerating, or outright lying.
Benny says, "I've got that same TV at home, but bigger"
Everyone else says, "Sure Benny"
Benny says, "... and then, after I f*cked her....."
Everyone else says "Sure Benny"
Everyone says, "who farted?"
Benny says, "not me"
Everyone says, "Sure Benny"