Taylor Swift to One Direction -- 'SHUT THE F**K UP!'

Taylor Swift doesn't just write breakup songs about her ex-boyfriends ... she talks SMACK about them during award shows ... and last night, she got caught. On camera.

It happened when One Direction took the stage to present a Moon Man at the MTV VMAs for Best Pop Video.

FYI -- Taylor used to date Harry Styles (they broke up in January) ... but apparently, she's still harboring some resentment -- 'cause as the band rambled on, the camera cut to Swift ... who leaned over to Selena Gomez and mouthed the words, "SHUT. THE. F**K. UP."

There are people out there who swear Taylor wasn't cussing -- that she was saying the words, "Sorry for my arm."

Those people are stupid.