It’s a used slang word from native reserves.

when you're hungover as shit after a spending a night drunk as a skunk

Only the monsters deadly natives know what bepsi truly is.

Bepsi is bepsi only the deadly natives know

A cola drink that has semen as it's main ingredient. Common in Colorado.

Bepsi, only danker

Bepsi is what Arabs call "Pepsi"(a soft soda drink) since theirs no P in their alphabet, so that's why they say "bepsi" instead of "Pepsi".