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1.
1. Greasy or low-budget food.

2. Very tasty but equally unhealthy food that you buy to eat because you're low on money.

3. Something a friend eats off the floor or out of the trash.
Speaking with the intent to grubish:
"I'm hungry but my funds are low, let's go get some grubish."

When caught eating something dropped on the floor:
"Eww grubish!!!"
by Grilled-Cheese November 14, 2012