George looks pretty tired after a long day at work to find Lynne vacuuming in her underwear. When Lynne fires up a conversation as soon as he's got through the door, George meets her question with disdain, knowing that it is undoubtedly a bad joke. George humours Lynne by going along with it, and because Lynne has her breasts exposed. Lynne completes her bad joke.
George does not know that Lynne has been thinking about making that joke all day, so much so that she forgot to get dressed.