From Wiktionary, the free dictionary
From àrd- (“high-”) + rìgh (“king”).
àrd-rìgh m (genitive singular àrd-rìgh, plural àrd-rìghrean)
- emperor, high king, monarch
- Edward Dwelly (1911) “àrd-rìgh”, in Faclair Gàidhlig gu Beurla le Dealbhan [The Illustrated Gaelic–English Dictionary][1], 10th edition, Edinburgh: Birlinn Limited, →ISBN