yawn       
 verb [ I ] uk/jɔːn/ us/jɑːn/
B1 打哈欠
to open the mouth wide and take a lot of air into the lungs and slowly send it out, usually when tired or bored
  I can't stop yawning - I must be tired. 我一个劲儿打哈欠——肯定是累了。 
 - More examples
 - He yawned loudly in the middle of her speech.
 - I was so tired, I couldn't help yawning.
 
yawn         
noun uk/jɔːn/ us/jɑːn/
 [ C ]哈欠 
an act of yawning
  Her eyes watered as she tried to stifle (= stop) a yawn. 她强忍着不打哈欠,眼泪都出来了。 
 a yawn[ S ] informal 乏味的事物(或人)
something or someone that is very boring
  The film was a total yawn. 这个电影太没劲了。