'mainstay' originates from nautical terminology, where 'main' referred to the main mast and 'stay' referred to a strong rope or cable that supports the mast.

'mainstay' was used in the 15th century to describe the rope supporting the main mast of a ship, and eventually became a metaphor for a central support in other contexts.

Initially, it meant 'a rope supporting the main mast of a ship,' but over time it evolved into its current meaning of 'a chief support or main element.'