doggy bag or doggie bag
n.
A bag for leftover food that a customer of a restaurant may take home after a meal.
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[From the assumption that such food would be given to the customer's dog.]
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2003. Published by Houghton Mifflin Company. All rights reserved.