ang (“iron”) + (m-)band (“prison”) From angmbandâ, gmb produces gb.
Sindarin
angband
place name. Iron Prison
Angband
noun. iron hell, prison
Angband
Angband
Angband
Iron Prison, Hell of Iron
grond
proper name. Hammer of the Underworld
The great mace of Morgoth glossed “Hammer of the Underworld” (S/154). It is simply grond “club” used as a name (Ety/RUD, PE17/183).
Conceptual Development: The name ᴱN. Grond first appeared in the Lays of Beleriand from the 1920s (LB/285) and again in Silmarillion drafts from the 1930s (LR/284), already with the translation given above. The name N. Grond appeared in The Etymologies with the derivation given above (Ety/RUD).
The fortress of Morgoth in the North, translated “Iron Prison” (WJ/15), a compound of ang “iron” and band “prison” (SA/ang, band).
Conceptual Development: The name G. Angband appeared in the earliest Lost Tales, but in these early stories it was translated “Hells of Iron” (LT2/77, GL/19). This translation persisted into Silmarillion drafts from the 1930s (SM/13, LR/259). However, N. Angband was translated “Iron Prison” in The Etymologies (Ety/MBAD), and this was the usual interpretation in Silmarillion revisions from the 1950s-60s (MR/305, WJ/15). Christopher Tolkien gave both translations in the Silmarillion Index (SI/Angband).