Madonna -- Everyone Chill ... I Was JOKING About Slutty Gala Outfit

Madonna's not ACTUALLY a slutty attention whore who planned to expose her nipples at the fanciest event in the world -- literally dressed like Leeloo from "The Fifth Element" -- at least that's what she's saying now ... claiming the photo she posted online Monday was a joke.

Sources close to the singer tell TMZ, she was just kidding when she posted the photo with the caption, "What i wanted to wear to Met Ball but Anna [Wintour] said Not this year! So I'm gonna work on music instead?"

Our sources say Madonna didn't even have plans to go to the Gala ... she was in the studio Monday night during the event and will stay there all week.

This just in: Madonna is 55 years old.