dull-witted       
adjective uk/ˌdʌlˈwɪt.ɪd/ us/ˌdʌlˈwɪt̬.ɪd/
 悟性低的; 理解力差的
slow to understand or learn; not very intelligent
  He was an unimaginative and dull-witted man. 他是一个缺乏想象力的人,悟性也不高。 
 They are devoted and dull-witted enough to obey any order. 他们既忠诚又愚蠢,这足以令其服从任何命令。 
 - More examples
 - He sounds rather dull-witted to me.
 - Under Professor B even the dull-witted students would show at least a faint sparkle of intelligence.
 - His brother was very charming but he was considered the dull-witted one with little charisma.
 - Optimism is easily mistaken for dull-witted naivete.
 - The show features a whole cast of Dickensian villains - dull-witted aristocrats, time-servers, corrupt spongers.