Other forms: mainstays
A mainstay is the thing everything else depends on. The mainstay of the economy, remains the mainstay, a mainstay of the team.
The word is nautical: the mainstay is the great rope holding the mainmast forward, and losing it brings the mast down. That is why the figurative sense means not merely important but load-bearing.
1nounmainstayThe thing everything else depends on.
“Fishing has long been the mainstay of the island's economy.”
mainstay is a support.
Main plus stay, from the rope supporting the mainmast of a ship.
Middle English
main stai
English, today
mainstay
From Middle English main stai, equivalent to main + stay.
Etymology from Wiktionary, CC BY-SA