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1.
A retort that means, "What you've just said is plainly not true, although I certainly wish it were."
"I'm never going to call or e-mail you again." "If only!"

"Aren't you worried about the Republicans? Once they get into office, they'll reduce the size of government." "If only!"
by Doctor Whom July 15, 2005
 
2.
it can be used to mean i wish this would happen. or it can also be used as a phrase rto mean maybe if this happens then....
person1: if only i were queen of the world.
person2: if only i was older than maybe he'd like me.
by purpleanjyl August 18, 2005
 
3.
A less assertive phrase to substitute for "as if", which was commonly used during the Valley Girl phase most frequently in the western United States. (sarcasm.)
Guy: You know you want to hook up with me!
Girl: {{prsh} If only! Keep troopin`.
by C_nonomous August 19, 2005