Nitwit (informal) (5)
(Other definitions for twerp that I've seen before include "Despicable person" , "ass" , "Silly-billy" , "dope" , "Silly person" .)
(Other definitions for ninny that I've seen before include "not a wise man" , "Stupid foolish person" , "gravely so miscalled by mechanicals" , "Fool" , "Silly billy" .)