A coloquial abbreviation of "As if I care", or "as if you care". Used to express disbeleif at the other's lack of consideration, or to express a lack of consideration of the other's predicament.
"i promise never to do it again"
"zif-i, you can do what you want, i don't care anymore"
"I hope you do well in your exam tomorrow"
"zif-u - you were totally against me going back to college"
"Hey how're you doing mate?"
"zif-u, you're full of shit"
"I've been getting these headaches every time you play your heavy metal music"
"zif-I, you don't think about me when you mow the lawn at 8am!"
"zif-U, you greasy haired mother"