E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One who gives moral but not material support. The allusion is to boxing or prize-fighting, where each combatant has a bottle-holder to wipe off blood, refresh with water, and do other services to encourage his man to persevere and win.
Lord Palmerston considered himself the bottle-holder of oppressed States . He was the steadfast partisan of constitutional liberty in every part of the world.The Times.