An exclamation which is uttered in order to convey the speaker's absolute boredom - usually caused by a situation they involuntarily find themselves in, eg a corporate meeting.
Humour is derived from the fact that yawning is an involuntary act, yet this phrase explicitly names the action rather than performs it - signifying that the speaker wants everyone to now they are extremely fucked off
Corporate Powerpoint Presentation Type : "So here we can see last years Net Promoter Score broken down by product area, product type and lastly, vendor. Going forward we are looking to move into a more diverse marketing space"
Normal Person : "Fucking YAWN!"