Said after something meaning it's the 100% truth. This cannot be used sarcastically, or any other -ly, except truthfully. You don't fuck with this shit.
It's origins come from South Park season 4 episode 16 'The Wacky Molestation Adventure.'
Person A: Holy shit, I just won $1,000,000 on this scratch ticket.
Person B: Bullshit.
Person A: No foolies.
Person B: Holy shit, buy me a car.
Person A: No.