A person with limited mental capacity who can perform tasks and think only like a young child, in medical circles meaning a person who lacks the capacity to develop beyond the mental age of a normal five to seven-year-old child (not commonly used today)
Imbecile means stupid. It was an imbecile thing to do. someone who is very stupid or behaves very stupidly = idiot (imbécile, from imbécile , from imbecillus, probably from baculum )
disapproval If you call someone an imbecile, you are showing that you think they are stupid or have done something stupid. I don't want to deal with these imbeciles any longer. = idiot