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。

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。

something or someone that is very boring

The film was a total yawn. 这个电影太没劲了。