Levity is behaviour that shows a tendency to treat serious matters in a non-serious way. At the time, Arnold had disapproved of such levity. = frivolity. lack of respect or seriousness when you are dealing with something serious gravity (levitas, from levis )
The quality of weighing less than something else of equal bulk; relative lightness, especially as shown by rising through, or floating upon, a contiguous substance; buoyancy; opposed to gravity