Mentally impaired. Someone who exhibits a low intellectual capacity. Also referred to as a retard(tard for short), mongoloid, or the commonly used medical term: window licker.
the word acts as a replacement for nouns and general abstract "things" you just can't remember their definitions. usually its not that there is no word to express the certain noun, for example, "chair", but at the moment of creating the grammer of the whole sentence, it slips out of your mind. that's where "Shlingy" comes along. Its resembles the English use of "Thingy" in a sentence.
1)
someone: Hey dude, can you pass me the shlingy?
dude: Which Shlingy, Salt Shlingy or katchup Shlingy?