1 foolish

2 stupid

3 expressionless

4 blank

5 to stay

1 to stare blankly

2 to be stunned

3 to be lost in thought

1 fool

2 sucker

1 dull

2 lifeless

3 sluggish

1 (loanword from Japanese "tennen boke") space cadet

2 muddleheaded

1 lit. dumb as a wooden chicken (idiom); fig. dumbstruck

1 to have a lifeless look in one's eyes (idiom)

1 lit. eye startled, mouth struck dumb (idiom); stunned

2 stupefied

1 dumbstruck (idiom); stupefied

2 stunned

1 to prevent a disaster before the event (idiom); to nip sth in the bud