An adjective originally coined by the professional medical community to denote an individual who was "delayed or held back" in perpetuity since birth, or shortly thereafter. Colloquially, it pejoratively assumes an individual was previously tarded at some point, and is now RE-tarded.
Tim: You just tried to skate down that hill with no wheels on your board, that's retarded man.
Tom: Piss off wankercheese, I've never even been tarded before in my life.
Tim: Well you are now asswaffle!!
A person WITHOUT any mental illnesses/disabilities who still manages to be the stupidest person in a room full of people diagnosed with mental illnesses/disabilities and/or a person who is immune to logic/common sense.
Retard: "Get the 4th dose, the first 3 doses of an experimental DNA-altering compound that provide 0 immunity or reduction in transmission isn't enough, the 4th one will protect you because the government said so, trust the government because they never lied or hide anything from the people"
Guy 1: What did that Guy say?
Guy 2: Don't worry about it, he's a retard