Two conditions:
1: Your herb bag is empty.
2: You are indoors with no ashtray.

So, you use the empty bag as an ashtray, known as the 'Ash Bag'
Josue: Hey man, where do I ash this?
Rick: Use the ASH BAG maaann

Jackie: Hey where's the ash tray
Jack: Use the Ash Bag!
Jackie: aww man, we're outta the herb?!?
Jack: Yes.
Josue: what?!?
by DiffrntStrokes March 13, 2011
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The fine dust that clings to the inside surfaces & bottom corners of a cocaine bag & any smooth surface used for snorting.
"There was so much Bag ash on the toilet cistern I pushed it all together & managed to get a full line"
by Crixxus November 7, 2019
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