How to say "dictionary" in Arabic

No Hebrew relative. Qāmūs originally meant 'the ocean, the deepest sea' — from the root ق-م-س, 'to dive, plunge underwater' (some scholars instead trace the word to Greek ōkeanós, though that theory is debated). Either way, the word came to mean 'dictionary' after al-Fīrūzābādī titled his 14th-century masterwork al-Qāmūs al-Muḥīṭ, 'The Encompassing Ocean' — a vast sea of words, and the name stuck for dictionaries generally.