Slang for 4-HO-MET, a lesser-known novel psychedelic of the Tryptamine class (which includes substances like melatonin, serotonin, DMT and Psilocin). Users typically describe Schleeb as a more recreational version of psilocybin mushrooms due to its less serious headspace and greater emphasis on visual effects.
John: "Yo Billy you tryna shnort this schleeb?"
Billy: "Hell yes John lets shnort this schleeb!"
The worst insult anyone could call you. It's meaning is something unspeakable, even in this eternal hellhole that is the internet. It's not even considered a slur, which only makes it worse, as the person saying it doesn't even consider you worth enough to sacrifice their profesionality just to insult you.