Khuzdul

Khuzdul, or Dwarvish, was the secret language of the Dwarves. Aulë, the creator of the Seven Fathers of the Dwarves, taught them "the language he had devised for them". Therefore, unlike many other tounges, Khuzdul is therefore unrelated in origin and different from the languages of the Elves, due to descending not from an Elvish tongue but from Aulë himself. Few of other race have ever succeeded in learning it. The Dwarves were not unwilling to teach Khuzdul to their close friends, but Men found it difficult to learn more than a few words, some of which were loaned. In the First Age, before the ancestors of the Edain crossed into Beleriand, they had contact with the Dwarves in Rhovanion and later in the Blue Mountains. As such, there are many similarities between Khuzdul and the native tongues of men, such as the language of the first and third houses of the Edain. That language was the ancestor of Adûnaic, the tongue of Númenor and the direct ancestor of the Common Speech. These Mannish languages displayed Khuzdul influences. During the Third Age the Dwarves of the Westlands had mostly adopted Westron and Khuzdul was a learned language, being taught to their children at an early age. They used it among themselves and as a language of books and lore, and for records not intended for outsiders. By their own will the Dwarves resisted the change of their language and the language diversified and changed so slowly, 'like the weathering of hard rock compared with the melting of snow' considering the languages even of the Elves. As a result, it remained similar to the original form taught by Aulë, and even if the sundered clans developed their own dialects, even in the Third Age, the clans could communicate easily, serving as a lingua franca between them.
Among themselves, the Dwarves spoke Khuzdul, their own language, or used Agla, a secret language of signs and gestures. Among other peoples, Dwarves adopted an outer language, as well as outer names (they never would reveal their true Khuzdul name to non-dwarves among Durin's Folk Rhovanian or Rhovanian Westron was the common outer speech, while the Firebeards spoke southern Westron or Dunlendish, and the Broadbeams northwestern Westron, Rhovanian or Labba. In the ancient days of the First Age both tribes had also used Sindarin as their outer language. The other four houses used the Easterling or Haradrim Languages though they would adapt names shaped after those of the three western tribes when they came to their lands.
One of the only major phrases known to outsiders is their battle-cry: Baruk Khazâd! Khazâd ai-mênu! meaning Axes of the Dwarves! The Dwarves are upon you!