Sling of Courage is a Shop Relic. It grants 2 strength at the beginning of elite fights. Also works in the Dead Adventurer and The Colosseum (second fight only) events. It does nothing against bosses.
This relic reference the tale of David and Goliath, in which the latter, usually regarded as gigantic man, was slain by the former, using only a stone and a sling.
The phrase David and Goliath has taken on a more popular meaning, usually denoting a situation where a smaller, weaker opponent faces a much bigger, stronger adversary.