ZAYN MALIK has been hanging out with a producer named NAUGHTY BOY since he left ONE DIRECTION. And during that time, Naughty Boy has been trading jabs with LOUIS TOMLINSON on Twitter. Zayn has stayed out of it. Until now.

Yesterday, Naughty Boy posted a photo of him and Zayn and captioned it, quote, "Replace this." The photo had an old-school looking filter applied to it.

So Louis Tweeted, quote, "Remember when you were 12 and you used to think those Mac filters for your pictures were cool? Haha! Some people still do. HA!"

That's when Zayn finally jumped in and said, quote, "Louis, remember when you had a life and stopped making [catty] comments about mine?" (Note: He actually said "b*tchy". So if that's cool with your audience, go for it.)

Louis hasn't responded yet. (Defamer.com has more of the prior back-and-forths between Louis and Naughty Boy.)