ˈyȯn, ˈyän. : to open the mouth wide and take a deep breath usually as an involuntary reaction to fatigue or boredom. yawn. 2 of 2 noun. : an opening of the mouth wide while taking a deep breath often as an involuntary reaction to fatigue or boredom.

Yawn comes from gionian in Old English, "open the mouth wide."

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to open the mouth somewhat involuntarily with a prolonged, deep inhalation and sighing or heavy exhalation, as from drowsiness or boredom. to open wide like a mouth.