Zaa~ko is a phrase which originates from YuzuSoft's game Tenshi Souzou Re-boot!. One of the main heroines, Tanikaze Amane teases her brother, a former demon lord who lost his power, by calling him a zako. It can be translated as a "small fry" or "prey", something really small and weak, like level 1 monsters. "Loser" is a close English equivalent.
