This word was pioneered by the infamous Brisbane suburban renegade "Hippy". This wretched vagrant and his frankly criminal exploits all over the suburbs these past few years has seen the slow spread of the term in the nomenclature of Brisbane's youth.
What exactly the term means remains unclear, and in fact, its very likely it means absolutely nothing at all.
The conclusion here is that the word was most likely coined by Hippy while he was Chewbaccin' (completely fucked, only able to mutter and grunt incoherent and vaguely Wookie-ish sounds). It seems he fluked the word 'schlank', and upon someone else having to recount the previous night's events, he decided he enjoyed the word and began using it to randomly replace other words in sentences.
Its most recognized form is an adverb in sentences organizing or describing being fucked up.
"Oi what up mad dog you wanna get on the schlank tonight or wat?"
"Those pills any good cunt?"
...
diapilated eyes, idiot grin "Fuck yeah bro I'm schlanked"
While cruising down the Valley on the back of a street sweeping machine after getting kicked out of the Mustang: "HAHAHA SCHLANKITY SCHLANK!"