beleg (“great, mighty”) + ost (“fortress”)
Belegostnoun. great fortress
belegostplace name. Great Fortress, Mickleburg, Mickleburg, (lit.) Great Fortress
Conceptual Development: The name G. Belegost appeared in the earliest Lost Tales, already with the etymology given above (LT2/230, LT2A/Belegost). In one place it appeared in an extended form Ost Belegost (LT2/244).
The name reappeared in Silmarillion drafts from the 1930s, where it was translated “Great Fortress” alongside its Dwarven name Gabilgathol (LR/274). The name N. Belegost was translated “Great City” in The Etymologies (Ety/BEL), where its decomposition was made explicit as Beleg-ost (EtyAC/OS).
Belegost (beleg + ost) was a Sindarin translation of the original Dwarvish name Gabilgathol and both mean "Great City". Unlike other names of the Silmarillion, the text also gives us an English rendering, which was possibly from Westron: Mickleburg. Mickle is a root meaning "big"; see also Michel Delving. The city's Khuzdul name Gabilgathol contains the elements gabil "great" and gathol "fortress". Túrosto was the name in Quenya for Belegost.