dullard

Now Captain Wellsby was no dullard and he purposed to make short work of these vile pirates.

She had no sooner said it than I was convicted of being a dullard.

I should have been a dullard, in sooth, had I not learned much in the companionship graciously granted me by my honoured mistress.

The quick mind supplies the apt repartee, while the dullard thinks of the appropriate reply next morning—if at all.