1) yiyf is a short version of letters combined. it means yoda is your father.
2) yiyf is also a short version of letters combined that mean yes it's your fault.
1) person 1 texts "who is your daddy?"
person 2 texts "yiyf"
2) person one texts "it isn't your fault"
Person two texts "yiyf"
Yogurt In Your Face!
Used when you want to aggressively want someone to either ingest or be splattered by yogurt. Comes up surprisingly often.
Person 1: What do you think you're doing over there?
Person 2: YIYF!
Person 1: Agh! WTF, YIMF!