This American deep-water tune was sung at the braces. It comes from Cohasset, Massachusetts sailor Nathaniel Silsbee who learned it at sea in the 1880s and sang it for his niece take down to paper in 1893. It is sometimes called a stamp-and-go shanty.
A stay is a rope that holds a mast upright, and a backstay runs from near the top of the mast toward the back of the ship.
When not in use, sails were furled high in the masts, tied with rope. Upon orders, a sailor or crew needed to climb the rope ladders and unfurl the sail. Coming down the backstays refers to the stunt of sliding down the sails or ropes, a quicker way to return to deck from high aloft. The act gives title to Caroline Tapley's 1974 book, John Come Down the Backstay.