1. You Fucking Idiot.
2. Interchangeable with FYI (For Your Information), with either a more comical or more offensive overtone.
3. Deliberate mistake instead of FYI, especially when preceded by "an" instead of "a". Designed to look like a typo of FYI but really a stealth way of telling someone they're very stupid, or that you're seriously pissed off with them. This usage is the special forces of abbreviation, giving all the fire-power of a WTF (What The Fuck), while leaving none of the evidence.
Dude, just an YFI... smoking kills.
Dear Mr. Taxman, just an YFI, I hold my savings Offshore so legally I'm not obliged to declare them to you.
Dear Mr. Officer, just an YFI, this is Amsterdam and it is illegal for you to bust my ass for smoking this rather sumptuous marijuana cigarette in front of you. Want a toke?
Husband: "Honey, I don't want to make love to you this weekend because you've been a nagging bitch: just YFI, it's not because I am cheating on you, impotent or gay."
Wife: "How many times do I have to tell you... it's FYI, not YFI."
Husband: "Oh, right." (Goes to fridge and gets himself a cold beer to celebrate the priceless moment.)