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Machinery. a supporting or checking piece in a mechanism.
Building Trades. an anchored tension member, as a cable, permanently or temporarily supporting a compression member, as a tower or pole, subject to a pull above its base from the opposite direction.
a strip of leather at the back of a shoe used for reinforcement and sometimes to connect the quarters.
Nautical. any of various shrouds forming part of a vessel’s standing rigging and leading aft from masts above a lower mast to the sides or stern of the vessel in order to reinforce the masts against forward pull.
Historical Examples

I accordingly slung the glass over my shoulder, swung myself off the yard on to the backstay, and so descended to the deck.
A Pirate of the Caribbees Harry Collingwood

In my fall I grappled with the backstay, and brought myself up, and landed on the cross-trees.
Yorkshire Oddities, Incidents and Strange Events S. Baring-Gould

I felt at times willing to quit my feeble hold of a backstay or shroud, and seek repose by diving into the briny billows beneath.
Jack in the Forecastle John Sherburne Sleeper

It is kept steady by a backstay on each side of the foretopmast-stay.
Every Boy’s Book: A Complete Encyclopdia of Sports and Amusements Various

Cut away a brace or a backstay, now, aboard that Frenchman, and away would go a whole heap of his canvas.
Under the Meteor Flag Harry Collingwood

They climbed everywhere, up or down, on a sail or its leach, a single rope or a backstay.
The Grain Ship Morgan Robertson

He kept his clutch on the backstay with the dizzy notion that this saved him from clutching some one’s throat.
The Skipper and the Skipped Holman Day

But Ulysses lashed the keel to the mast with the backstay, and on these he sat, borne by the winds across the sea.
Stories of the Old world Alfred John Church

A piece of iron used instead of a chain to confine the dead-eye of the backstay to the after-channel.
The Sailor’s Word-Book William Henry Smyth

Just as he spoke, Tommy Rebow was hunting the animal from shroud to backstay, up over the mast-head and down again.
Sunshine Bill W H G Kingston

noun
(nautical) a stay leading aft from the upper part of a mast to the deck or stern
(machinery) a supporting piece or arresting part
anything that supports or strengthens the back of something, such as leather covering the back seam of a shoe