It comes from the roots uber and disgusting. It means is the most disgusting it could possibly be, usually inducing vomit and or other evacuations of bodily fluid.
person 1: dude you see 2 girls 1 cup?
Person 2: yeah, that shit's ubsgusting!
-adjective
something causing disgust; that is offensive to the physical, or moral taste. Similar to the word disgusting; but something much worse, beyond any actual description.
That man is eating a tarantula? That is besgusting. I think green jello with carrots is besgusting.