Conjunction

1. as though; in a manner suggesting

   The old man stumbled, as if he were about to fall.

2. in mimicry of

   When the teacher's back was turned, the class clown would hold his stomach as if he were ill.

Interjection

1. referring to something that the speaker deems highly unlikely
   • 1903, Frank Morris, The Pit: A Story of Chicago, p 7, ISBN 1419177621.

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