“Brainrot” is a slang insult for users who are “too online.”
When you see this commented on TikToks, Youtube videos, or Tweets, it’s often a cutting way to imply that someone has lost sight of reality as a result of being too steeped in hyper specific internet cultures. As a result, they might have zany opinions or bizarre behaviors without even realizing that most people don’t think or act that way.
Person 1: “Yeah dude she totally rizzed up the skibidi toilet until it became a gyatt sigma male. Cringe much?”
Person 2 : “Dude, you have to get off TikTok. You’ve got brainrot.”