"Boi tf" or "boy the fuck" can mean BOI!!!!! or just boi what the fuck! Or more commonly used just to express how speechless you are about something you have been confused about for awhile, but now understand.
Hallie :I thought we were going to Tatum's house?
Tatum: No Hallie we aren't going to my house we are going to Kara's house.
Hallie: "boi tf" really? Ok! What time?
Tatum: 7:00 lmao! See you there!