Thimbalt was probably a fortress of Morgoth in a early version of the Legendarium, located near Angband on the slopes of Thangorodrim. The name only appears on the north part of the First 'Silmarillion' Map and nowhere else. Thimbalt was struck out using a pencil on the map.
While its meaning is unclear, Paul Strack suggests on Eldamo that it is a rejected Noldorin name.