The actress took to the stage at the Williamstown Theatre Festival Friday night to perform a remixed version of Swift's song with Ashley Park, who plays another Plastic on the Broadway version of "Mean Girls."
Park introduced the song as being for "all the Regina Georges out there."
In the remix, the lyrics are changed up slightly to reference popular "Mean Girls" quotes and scenes.
"Someday I'll be fetch enough so you can't hit me and all you're ever gonna be is mean," the pair sing.
(Warning: Video contains some NSFW language)
The pair continue, "You have pointed out my flaws again when you wrote it in your burn book."
Seyfried even pauses the song to fake cough and say, "I'm sick" – another "Mean Girls" reference that received a big cheer from the crowd – before resuming the song.
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This article originally appeared on USA TODAY: Taylor Swift's 'Mean' gets 'Mean Girls' remix from Amanda Seyfried