A waxy, petroleum-based protective coating used to prevent rust and corrosion on metal surfaces.
From 'cosmo-' (perhaps related to cosmetics, things that cover/adorn) + '-line' (perhaps from 'line' or chemical suffix), a trademark that became a generic term.
Cosmoline was the military's favorite rust preventative for decades—soldiers deployed to humid jungles or arctic wastelands had to spend hours carefully cleaning it off weapons before use, making it both beloved and despised in armed forces worldwide.
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