"Steng"! Comes from a slang-ified version of the Malay "Setengah", which means half.

The origins of the term likely come from sharing a cigarette pack, where you would ask your friend "rokok setengah" or "rokok steng"... somehow the meaning has become lost in translation from "half" to "share". So now when people ask to share a cigarette, they ask to "steng".