KATY PERRY seemed to be joking when she said she broke off that kiss with MILEY CYRUS because, quote, "God knows where that tongue has been." 


But it sounds like Miley didn't take it that way . . . and as a result, IT IS ON.  Because she fired back with a Tweet that insulted both Katy and her ex-boyfriend JOHN MAYER.


She said, quote, "Girl if you're worried about where tongues have been good thing ur ex boo is ur EX BOO cause we ALL know where THAT [tongue's] been."


She might have tried to smooth things over, though, because her next Tweet was a picture someone had DRAWN of the two of them kissing, and Miley captioned it, quote, "Don't you act like you didn't love it."



And it sounds like Katy is taking all of this in stride, because she replied, quote, "Oooo gurrrl I'm gonna give you the BIGGEST spanking when I see you in the UK."


For the record, John Mayer's tongue has been on or in the following ladies:  Jennifer Love HewittVanessa CarltonRhona MitraJessica "Sexual Napalm" SimpsonCameron DiazMinka KellyJennifer Aniston and Taylor Swift.