How old is the expression "as if"?
Solution 1:
The OED dates it to circa 1903, from the book The Pit: A Story of Chicago by Frank Norris, with this citation :
'Maybe he'll come up and speak to us.’ ‘Oh, as if!’ contradicted Laura.
The Merriam-Webster reference is probably to the Old English use of "as if" in in the sense 'as the case would be if'.
Solution 2:
Merriam-Webster records its first use in the 13th century.