n.

1. 【船】支持主桅的绳索
2. 唯一的依靠；支柱
   In actuality, teachers, civil servants, and military personnel are the mainstay of our society.

   其实，军公教才是一个国家的支柱。

n.

1. a stay which extends from the maintop to the foot of the foremast of a sailing ship.
2. the chief support or basis:
   sugar is the mainstay of the island's economy.