When a person is the absolute most reliable, supportive member of a team or a family, they are called the "mainstay." This is a purely architectural naval term! On a massive sailing galleon, the tallest and heaviest wooden pole was the mainmast. To keep this massive mast from snapping and falling backward during a violent hurricane, it was supported by a single, incredibly thick diagonal hemp rope known as the "main-stay." If the mainstay snapped, the entire ship would physically collapse. Because it was the ultimate source of support, the word evolved to describe highly dependable humans!
