An offensive word used against disabled people, either able-bodied or not. This term is used against mentally and/or physically handicapped people. Such as people in wheelchairs, people with crutches and people with down syndrome or people with autism, dyslexia and other neurological disorders.
This word is often used on the internet by ableist 13 year olds who don't know any better.
13 year old boy on Youtube: retard
An actual smart person: Don't say that, it's offensive and used against disabled people.