Popularized by legendary Dad Boner Fred Eichler in the video "Archery moose down in 20 seconds", glassing's original meaning was to look through binoculars (glasses) while on an out-of-doors expedition.

As a part of Mommyspeak, "glassin'" can mean "looking", "hanging out", "chilling", or pretty much anything that the person using the word desires it to mean in context.