Ret/ard comes from the term mental ret/ardation of which there's a connection between autism and MR. It does mean idiot. It means "mentally incapable". It means idiot in the most derogatory form of the word. https://twitter.com/SeleDreamsFren/status/1245807464014065670

The amount of pick me hehe it just means idiot :3 under this thread oh boy you're young and didn't consciously grow up where everyone was using it in the form of a slur towards us, huh