Stands for "For Your Ass(es)/Fo' Yo' Ass(es)," but basically means "for yourself/yourselves." It's a way to call attention to--or remind someone of--something.
F.y.a. is my own play off of corporate-speak's "f.y.i" (for your information).
I thought the thought (and act) of fresh grads/entry-level co-workers plugging "f.y.i." into their communication as much as possible in order to feel important and part of the corporate world is funny. (F.y.a., I did it, too, for a minute.)
Every day use:
"F.y.a., make sure to bring back some <food> from <restaurant>. And hurry. I'm hungrier than a moph!"
"F.y.a. to all the biters, don't forget when and where this phrase came from. For everyone else, enjoy. Tim-Ass."
Casual office use:
"F.y.a., Lumberg's meeting with the client got cancelled. So we probably won't need those TPS reports by E.O.D. anymore."