The fictional people whose language Tolkien called Khuzdul are the Dwarves.
Khuzdul is the secret language of the Dwarves in Tolkien’s legendarium. Tolkien gave the language a Semitic-inspired sound and structure, setting it apart from the Elvish languages that dominate many of his stories. Dwarves generally guard Khuzdul carefully and rarely teach it to outsiders.
The Dwarves use their own language alongside languages learned from neighboring peoples. In The Lord of the Rings, the battle cry “Baruk Khazâd! Khazâd ai-mênu!” is translated as “Axes of the Dwarves! The Dwarves are upon you!” The name Khazad-dûm also belongs to Khuzdul and means Dwarrowdelf in Tolkien’s English gloss.
Readers sometimes confuse Khuzdul with the language of the Orcs or with Sindarin, which is spoken by Elves. Tolkien created both vocabulary and linguistic history for Khuzdul, but he published far less of it than Quenya or Sindarin.
Its secrecy is therefore both a cultural feature in the fiction and a reason the surviving vocabulary is comparatively limited.