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The messy, glorious act of making or getting a Big Mac so big it looks like it might explode. Used mostly by people who work at McD's and don’t care if they get fired.
I just did a mactastic and now my hands smell like ketchup and regret.
My lunch break is a mactastic. It's the only way I survive this job.
I did a mactastic so fast the cashier looked at me like I was a monster.