sim·ple·ton 
/ˈsɪmpəltən/ noun
plural sim·ple·tons /ˈsɪmpəltən/ noun [count] old-fashioned : someone who is not very intelligent or who does not have or show good sense or judgment
▪ The instructions were so complicated I felt like a complete simpleton.
▪ The instructions were so complicated I felt like a complete simpleton.
