Nerek

Nerek (ネレク, Nereku), sometimes referred to as Wing Water (翼水, Tsubasa mizu) is a substance used by the Winged Horde.

Information
Nerek is a foul-smelling liquid used by the Winged Horde to protect their wings and repel insects. Without this substance, their wings tend to be attacked by parasites. As the concoction is made from fish heads, it often carries a strong smell. While the Winged Horde are generally used to it, the odor tends to bother others.

It was generally referred to as Wing Water by non-Wingers.