A condition where, at the peak of a big yawn, one suffers a few seconds of deafness due to the opening and closing of the eustachian tubes connecting the ear and throat. Opportunists who know of another's yawn deafness use this condition to say something to the person yawning that they don't want them to hear - usually an insult, but spoken under breath or in a normal
conversational tone. Better practice is to do it with friends present who will all hear
what you said. A sizeable yawn can open up a good 5 seconds of deafness
opportunity.
NOTE: Not everyone suffers yawn deafness, so there is always a risk when you try it on someone new.
Ken: Man ... that was a hard day's work (starts yawning)
Fred: I came twice in your wife's arse
last night.
Ken: (finishes yawning) Sorry - yawn deafness ... what was that?
Fred:
Nothing ... I was just agreeing with you.
Hard day indeed.