Kylie Jenner's Ass Tattoo Is Crazy

Kylie Jenner's new booty ink is insane -- at least if you're taking it literally. Kylie now has the words "before sanity" inked in red across her buttcheek ... TMZ has learned.

Our sources tell us Kylie got the urge to add some new meaning to an old tattoo Saturday, so she rang up celebrity tattoo artist Rafael Valdez. He added the word "before" in front of the "sanity" tattoo Kylie got last year.

Valdez gave Ky her first tattoo, a tiny red heart. He's also Tyga's personal tattoo artist and has inked everyone from Iggy Azalea to Johnny Manziel.

The tiny tat didn't come cheap ... Valdez charges $200/hour with a $600 minimum, according to his site.

It's unclear exactly what the meaning behind Kylie's behind tattoo is -- but considering the placement -- maybe it's a metaphor?