As if he's ever going to get a girlfriend!And what you mean is that he hasn't a hope in hell of getting a girlfriend. You can also use it on its own, like in the film Clueless: As if!
But like all colloquial phrases, its usage is shifting. I noticed that I use it like this:
As if I know what we're doing yet.
As if I have sat nav.It signifies something that we both know isn't true and is a faintly ridiculous suggestion. I'm almost scoffing at my interlocutor for suggesting it. My sister, however, uses it like this:
As if you can't eat bread and stuff?
As if he doesn’t pass that on, does he even know?It doesn't help that both the examples I could find in emails from her contain a negative element, whereas mine are affirmative. But she's definitely not scoffing; she's more just exclaiming and engaging in mutual feeling (so she's sympathising in the first one, and joining me in being outraged in the second).
But the main difference is that the propositions are true, in hers, but mine are false. As if I have sat nav means (It's not true that) I have sat nav, but As if he doesn't pass that on doesn't mean (It's not true that) he doesn't pass that on. It was said when he actually didn't pass it on. It's gone from being a negator (the grammaticality of yet in the first example proves it is, as you can't have yet in an affirmative sentence) to being an intensifier of sorts, used to make the proposition it's attached to more emphatic.
Urban Dictionary says that you can use it just to draw attention to something (true):
Cassie: Are you going to LeedsFest this year?This is even more extreme, as in this exchange as if is attached to new information which has not previously been mentioned/known by both parties. In all of the other cases, it's attached to given (recently discussed or already-known) information (echoed information, often, so that the proposition has just been uttered by the other person before the as if expression that echoes it). In this last example, it's attached to the answer to a question asked by 'Cassie', who does not yet know the answer, so it must necessarily be new information.
Jordyn: As if I can't go to LeedsFest! HATE THAT!