WBF, short for Worst Best Friend. A friend that you like a lot but they always flake or let you down. Phonetic play on WTF, and to be used as such, like an exclamation, when appropriate.
Kristy: "Franke just texted, he can't come to the party."
Dan: "WBF! Why does he even pretend like he's gonna go?"
Kristy: "Seriously, W-B-F."
Brian: "Hey guys thanks for helping me move -- wait where's Dan, I thought he said he was coming?"
Franke: "I think he's sick or something."
Brian: "WBF! He's been sick for like 10 years, no one's buying that shit anymore."