overearnest       
adjective (also over-earnest) uk/ˌəʊ.vərˈɜː.nɪst/ us/ˌoʊ.vɚˈɝː.nɪst/
 过分认真;过分严肃
too serious and determined, especially so serious that you are unable to find your own actions funny
  She's an over-earnest director who's made a handful of second-rate films.
 His novels could sometimes be over-earnest. 他的小说有时可能过于严肃。 
 - More examples
 - An over-earnest presenter was asking her increasingly dull questions.
 - I got stuck talking to an overearnest academic.
 - She plays the over-earnest daughter of a British family visiting the Dordogne.
 - Critics describe his plays as over-earnest.
 - The overearnest dancer droned on about her partner for so long that I wanted to fall asleep.