To be chilling, doing nothing, just like moss, the commonly known fungus that just chills out on rocks.
Origin is believed to be Pickering, Ontario, Canada, a city that is part of the Greater Toronto Area.
Aldo: What it do, player?
Brianna: You know, just mossin'.
the act of relaxing and being lazy; basically doing nothing
I've been mossin' on the couch for hours