imbecile

The definition of an imbecile is someone who is foolish or isn’t very smart.

(noun)

An example of an imbecile is someone who treats people badly and does stupid things that don't make any sense.

See imbecile in Webster's New World College Dictionary

noun

Obsolete a retarded person mentally equal to a child between three and eight years old
a very foolish or stupid person

Origin: Fr imbécile < L imbecilis, imbecillus, feeble, weak, prob. < in-, without + baculus, staff (see bacillus): hence “without support”

adjective

very foolish or stupid

See imbecile in American Heritage Dictionary 4

noun

A stupid or silly person; a dolt.
A person whose mental acumen is well below par.
A person of moderate to severe mental retardation having a mental age of from three to seven years and generally being capable of some degree of communication and performance of simple tasks under supervision. The term belongs to a classification system no longer in use and is now considered offensive.

adjective also im·be·cil·ic (ĭmˌbə-sĭlˈĭk)