From Tolkien Gateway

zigil is a word in Khuzdul, occurring as an element in Zirakzigil, the Dwarves' name for the mountain called Celebdil or Silvertine by other races, and in Zigil-nâd, a rejected Dwarvish name for Kibil-nâla, the river known to the Elves as Celebrant and to Men as the Silverlode.

Zigil therefore evidently means ‘silver’.