mainstay

Etymology

From Middle English main stai, equivalent to main + stay (“rope”).

noun

1. A chief support.
   Agriculture is the mainstay of this country’s economy.
   At the other extreme, we have that true maid-of-all-work type, the 0-6-0, the mainstay of goods, shunting, and often of passenger work right from the earliest days to the present time. 1945 November and December, H. C. Casserley, “Random Reflections on British Locomotive Types—1”, in Railway Magazine, page 319
   As with most other railways, freight revenue is the mainstay of the balance sheet. In Canada, passenger revenue is only about one-tenth that of freight. 1959 November, J. N. Westwood, “The Railways of Canada”, in Trains Illustrated, page 555
   Oil is the mainstay of Nigeria's economy. 2000, Jedrzej George Frynas, Oil in Nigeria
   Conventional radiography has a major role in, and remains the mainstay of, initial evaluation and follow-up of rheumatologic disease. 2014, Marc C. Hochberg, Alan J. Silman, Josef S. Smolen, Rheumatology, page 307
2. Someone or something that can be depended on to make a regular contribution.
   On the Bishops Stortford line, the crisis now seems to be over; the units designed for this service are the mainstay of the workings once again and although some of the inner suburban sets are still seen, very few L.T.S. Line units are noticeable. 1963 January, “Motive power miscellany”, in Modern Railways, page 65
   Like show dogs, dog actors became a mainstay in European and American contexts from the early nineteenth century with the convergence of public sentiment for dogs and popular interest in training them. 2004, Susan McHugh, Dog
   Crickets are a mainstay of panfishing with live bait—and a mainstay of bait shops—but they come off the hook easily and you'll be plagued by minnows and tiny fish constantly stealing your bait. 2010, Lamar Underwood, 1001 Fishing Tips
   X-Men: Apocalypse, directed by series mainstay Bryan Singer, gives Magneto, the Holocaust survivor who can control magnetic fields, and Xavier, the paraplegic telepath who tends to come off as really smug, next-to-zero shared screen time. May 23, 2016, Ignatiy Vishnevetsky, “Apocalypse pits the strengths of the X-Men series against the weaknesses”, in The Onion AV Club
3. (nautical) A stabilising rope from high on the mainmast to the base of the foremast.