Definition of retarded in US English:

retarded

adjective

offensive, dated
  • 1Less advanced in mental, physical, or social development than is usual for one's age.

    1. 1.1informal Very foolish or stupid.
      ‘in retrospect, it was a totally retarded idea’

Usage

In the sense ‘less advanced in development than is usual for one's age’, retarded dates from the late 19th century. It has acquired offensive connotations in recent decades, and terms such as developmentally disabled or having intellectual disabilities are now preferred

Pronunciation

retarded

/rəˈtärdəd//rəˈtɑrdəd/