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A Tim Stack is when you stack all of your Tim Horton's coffee cups, so that they take up less space in the garbage can.

This normally takes place in a classroom or office setting, where many people have to share the same garbage.

A Tim Stack will mainly be found in Canada, however they have been spotted in other countries under another other alias;
Starbuck Stack
Teacher: Hey class, can we start a Tim Stack in the back there? The garbage is beginning to overflow.
by ESL 2009 December 08, 2009