.Fly is a status effect in the series. It is only caused by the enemy-exclusive ability Bael's Curse. Characters afflicted are turned into flies, which greatly diminishes their stats and makes them weak to Expel. This status cannot be cured, once inflicted, the only way to reverse it is by killing and reviving the character.
In the original version of Shin Megami Tensei III: Nocturne, this status effect is Curse-type, but in the later version it is Death-type instead.