We use this expression to say that something is extremely unlikely or even impossible. It’s short for ‚As if that were possible.‘
You can use this phrase in ………… language.
A: It’s Honza’s birthday. Are you going to give him something?
B: As if! You know I hate him.
A: Do you think Pavla will be here early tomorrow?
B: As if! She’s never early, and the traffic is still bad after the floods!