Short for "as if that was true," "as if that would really happen," or "as if she'd really agree to that."
Basically, it means it'll be a cold day in
hell that the idea or event is true or ever will be. It's a hipper version of the 80's
expression, "Yeah, right!"
Enriched after selling plasma, old impoverished Nathan approached young,
smokin' hot Angela and asked her if she'd like to have dinner with him at the all-you-can-eat buffet: his treat. After
throwing up a bit in her mouth, she rolled her
beautiful blue eyes and blurted, "As if!"