Frivolity, lack of appropriate seriousness.
The Puritan elders tried to ban levity of all sorts from the community's meetings.
Levity originally was thought to be a physical force exactly like gravity but pulling in the opposite direction. Even as late as the last century, scientists were arguing about its existence. But today, levity refers to lightness in manner. This was once regarded as almost sinful, so the word has an old-fashioned ring to it and is usually used in half-serious tone of disapproval.