Another way to say "you'reboring" or "low-class". It's become popular recently largely because of its usage by the character Eleanor on The Good Place.
Basic Ben: Hey Jen, what if the deep state brainwashed us all into forgetting that cereal is just soup, but the broth is milk?
Jen: Just stop, Ben. Please. For your own sake. Ya basic!
Phrase from 80s song from Klymaxx "Meeting in the ladies room", meaning a woman stepping down from her classy status to put the beat down on some skanks trying to get her man.
"I’d hate to come down to their level and become a BW A basicwoman But if they don’t stop it’s gonna get scandalous "