Scarlett Johansson and Lindsay Lohan war!

Their feud has been simmering since 2006 when Lilo reportedly scrawled some funk about Scar Jo on a bathroom stall in a chi-chi dive bar in Manhattan's Lower East Side.

Scar Jo never responded publically until now at the Mean Girls toiletry graphitti diss until now.

"I don't know what the motivation was," Johansson says in a new interview.

'I remember it was something really vulgar - I mean shockingly so, like, "Whoa, what, who are you?"

Johansson then added she's only met the sapphic Sam gal pal only "like three times."

WTF?!  Stay tuned…