The first dragon (S/116), also called the “Gold-worm” (UT/75). The initial element of his name is clearly glaur “gold”, but the meaning of the second element is unclear.
Conceptual Development: In the earliest Lost Tales, his name was G. Glorund (LT2/19), appearing in the Gnomish Lexicon as Glorunn, a combination of G. glôr “gold” and an untranslated word gunn (GL/40). The word gunn does not appear elsewhere in the Gnomish Lexicon, but gund appears with the gloss “dragon” in Early Noldorin word lists as an element in the name Glorund (PE13/144, 145, 162).
The name Glorund was revised to Glómund in Lays of Beleriand from the 1920s (LB/208), and kept this form in Silmarillion drafts from the 1930s (SM/32, LR/255). The name Glaurung emerged in Silmarillion revisions from the 1950s-60s (WJ/46, 121). In these later forms of the name, the meaning of the final element is unclear.
prop. n. Tolkien notes "Alter Glaurung > Angruin". FAngruin