A Humangalong Dongalong (hu-mun-guh-long donga-long) is another word for a joint, spliff, doobie, etc. and is largely considered as one of the most fun things to say
Hey Otto, wanna strap up another humangalong dongalong?
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.