noun
big yawn
A large, noticeable yawn, often implying boredom or sleepiness.
会議中に大あくびをしてしまった。
I let out a big yawn during the meeting.
彼は大あくびをしながら起きてきた。
He got up while giving a huge yawn.
あくび is a general yawn; 大あくび specifically emphasizes a large, conspicuous yawn.
Compound of 大 (おお, 'big') and あくび ('yawn'). The reading おおあくび follows standard rendaku rules.