mentally handicapped - definition and synonyms

adjective old-fashioned 

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/ˌment(ə)li ˈhændɪˌkæpt/
  1. someone who is mentally handicapped is not able to learn or develop skills at the same rate as most other people because they have a problem with their brain. It is now considered more polite to say that someone is learning disabled or that they have special needs or learning difficulties.