Lindsay Lohan wasn't too pleased with Justin Bieber's Instagram rant, in which he poked fun at her tax problems. She also is assuming he hasn't hit puberty yet.

Lohan was reportedly "appalled" at Bieber's post, which he quickly deleted -- but which screenshots preserved forever.

Bieber, after a scuffle with a paparazzo, a hospital stint, a couple car wrecks (that actually, unlike LiLo, really weren't his fault) and a really crappy birthday, came under speculation that he was falling from grace a la Lohan, which inspired his Instagram denial and diss.

Still, Lilo is telling pals that it was uncalled for -- and that "karma will bite him in the ass."

The troubled starlet says she's learned to "brush off the negativity," likely because he's not within punching range, and she "chooses not worry about rude, ungrateful, obnoxious little hairless boys." (Admit it -- that made you laugh.)

Lately she has been pretty busy worrying about hairy DJs and boy band members. And, you know, about keeping herself out of jail.