Abbreviation for 'I Fucking Hate Your Guts Right Now.' However when you say, "IFHYGRN," you let the person you said that to know it means, 'I finally heightened your grenade running notification,' in which putting a halt to whatever made you mad enough to want to say that you hate their guts, and making the person who made you mad feel like they owe you one.
Man ONE: "So I fucked your mom last night."
Man TWO: "But she's dead..."
Man ONE: "Yeah I know. It was hell digging up her grave."
Man TWO: "IFHYGRN."
Man ONE: "So like, what does that mean?"
Man TWO: "Ah! You're such a n00b! It means I finally heightened your grenade running notification."
Man ONE: "Wow! Are you kidding me?! Sheesh! I owe you one!"
Man TWO: (*Laughs evilly*)