1. Large dark nebulae or dust clouds visible against the bright background of the Milky Way, appearing as dark patches in the sky.
Usage: astronomy; plural form; singular is 'coalsack'; most commonly refers to the Coalsack Nebula in the southern sky
2. Large sacks or bags used historically for storing or transporting coal.
Usage: historical; plural form

Examples

• Astronomers use telescopes to study the coalsacks and other dark nebulae in our galaxy.
• The Coalsack is one of the most prominent coalsacks visible from the Southern Hemisphere.
• Workers loaded the coalsacks onto the ship for transport to the factory.
• The dark coalsacks in the night sky are caused by dust blocking starlight.
• Early miners stored coal in large coalsacks before modern containers were invented.