【Simple】 ,  【foolish】 ,  【silly】 ,  【fatuous】 ,  【asinine】  can all mean actually or apparently deficient in intelligence.

【Simple】 , when it implies actual deficiency in intelligence, is applied chiefly to persons whose intelligence is that of a child and who are incapable of dealing with ideas or situations that involve much mental effort. It may imply either illiteracy coupled with a lack of native shrewdness, or feeblemindedness that does not amount to imbecility.

When used as a term of criticism of normal persons or their acts it suggests little more than failure to use one’s intelligence.

【Foolish】  (see also  【FOOLISH】 2 ) as a term of criticism of normal persons and their acts is stronger than  【simple】 because it imputes either the appearance of idiocy or imbecility or a want of intelligence or of good judgment that makes one blind to dangers or consequences.

【Silly】 is applied to persons who, though not mentally deficient in a technical sense, fail to act as rationally guided beings either by showing a lack of common sense or ordinary good judgment or by behaving in a manner that makes them ridiculous in the eyes of others.

【Fatuous】 does not often imply a pathological lack of intelligence, but it does imply an appearance of this and regularly suggests a combination of foolishness, stupidity, and inanity. It is a term of contempt rather than of impersonal description, and is capable of additional connotations (as fatheadedness, vacuousness, obtuseness, or loss of a sense of proportion).

【Asinine】 is also a term of contempt; it suggests an intelligence comparable to that of a donkey, considered the stubbornest and most stupid of the beasts of burden. As applied to persons or their acts, choices, or opinions it connotes an utter failure to exercise intelligence or thinking unworthy of a rational being.