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1.
A dirty bag is marijuana-smoking device - an alternative name for a lung... generally a 2 litre soft drinks bottle with the bottom cut off, and a bread bag sellotaped around the bottom.

Burning the marijuana on a foil gauze on top of the bottle whilst drawing the bag downwards causes smoke to collect in the bottle and bag. You then remove the gauze and suck the contents.

Multiple useage causes a brown residue to collect inside the device - hence 'dirty bag'.

In some semi-rural communities, disparate groups of dope smokers have been known to share a dirty bag, which is stashed in a particular bush or under a bridge for communal usage by whoever is passing by.
"Get us a bottle and a bread bag and some tape and we'll make ourselves a dirty bag"

"Fire up the d-bag mate"

"Did we leave the dirty bag behind that hedge?"
by Loomis McCartney August 20, 2008
 
2.
A "dirty bag" technically defines bagged (as opposed to canned) whipped cream. However, the term can also be used to describe a mug of hot chocolate with bagged whipped cream in it.
"Hey, James, can you bring me the dirty bag from the fridge?" -(bagged whipped cream)

"Ben, can you heat up some water and make me a dirty bag with extra chocolate?" -(hot chocolate with whipped cream)

"Hey, Kayla, bring me a dirty bag with extra dirty." -(hot chocolate with extra whipped cream)

"What, you mean the dirty bag?"
by TheDirtyBagga November 29, 2008