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Is this really a thing?
There is a notion called Brownbagging
which is, in the words of Wikipedia, "Consuming an alcoholic drink while concealed, usually, but not necessarily, in a brown paper bag, so as to drink in public where such activities are prohibited by law."
To an outside observer, it may appear as though the person is drinking from a bag, but they are actually drinking from a bottle that is concealed by the bag.
I couldn't find any trope written for it, but I'm sure I've seen it in media a few times — it can imply alcohol use without actually saying it as a Parental Bonus or Getting Crap Past the Radar.
Edited by KD
You drink out of a bag instead of a cup.