Definition of "as if"

I remember him as if it were yesterday, as he came plodding to the inn door, his sea-chest following behind him in a hand-barrow — a tall, strong, heavy, nut-brown man, […]

1881–1882, Robert Louis Stevenson, Treasure Island, London, Paris: Cassell & Company, published 14 November 1883

“ […] They talk of you as if you were Croesus—and I expect the beggars sponge on you unconscionably.” And Vickers launched forth into a tirade very different from his platform utterances. He spoke with extreme contempt of the dense stupidity exhibited on all occasions by the working classes.

1918, W[illiam] B[abington] Maxwell, chapter XXV, in The Mirror and the Lamp, Indianapolis, Ind.: The Bobbs-Merrill Company, page 203

Eustace gaped at him in amazement. When his urbanity dropped away from him, as now, he had an innocence of expression which was almost infantile. It was as if the world had never touched him at all.

1963, Margery Allingham, chapter 9, in The China Governess‎

Scotland needed a victory by eight points to have a realistic chance of progressing to the knock-out stages, and for long periods of a ferocious contest looked as if they might pull it off.

2011 October 1, Tom Fordyce, “Rugby World Cup 2011: England 16-12 Scotland”, in BBC Sport‎

“Better wait, hadn't you, Laura,” said Aunt Wess’, “and see. Maybe he'll come up and speak to us.” “Oh, as if!” contradicted Laura.

1903, Frank Norris, The Pit: A Story of Chicago, Sydney: Angus & Robertson, page 10

Ooo! Get off of me! Uh, AS IF!

1995, Amy Heckerling, Clueless, spoken by Cher (Alicia Silverstone)