From the root د ر ع (d r ʕ). Compare Ge'ez ድርዕ (dərʿ, “coat of mail, harness”), Tigre ደርዕ (därʿ), Tigrinya ድርዒ (dərʿi), Amharic ዲር (dir), ድር (dər), Soqotri daraḥ (“to put on (a garment)”). Nöldeke thinks that the Arabic is borrowed from Ge'ez.
Practice speaking and memorizing "درع" and many other words and sentences in Arabic.