The shortened version of Zirakzigil, the Dwarvish name for the Mountain of Moria on which Durin's Tower stood. The mountain's full name meant 'silver spike' or 'silver pinnacle', and the same meaning is found in its Elvish and Common forms: Celebdil and 'Silvertine' respectively. There is some confusion over the meaning of the individual elements of the name Zirakzigil, and hence over the precise interpretation of the short form Zirak. At one time Tolkien defined zirak as a Dwarvish word for 'silver', but he later seems to have reconsidered this, making zigil 'silver' and zirak 'a narrow peak or pinnacle'. Zirak also appears as the surname of the Dwarven smith Gamil Zirak, which we're presumably intended to take in a broader sense of 'tall' or 'high'.1
Given Gamil's fame as a smith, a connection to the metal silver seems to make rather more sense, so perhaps Tolkien named this Dwarf before he reconsidered the meaning of the word zirak.