A haven at the mouth of the river Brithon in Beleriand (S/120), a combination of the river name with bâr “home” (SA/brith, bar), hence “✱Brithon Home”.
Conceptual Development: This name appeared in the Silmarillion drafts from the 1930s (SM/227, LR/180). In The Etymologies, the name Ilk. Brithombar was designated Ilkorin (Ety/BIRÍT), with a derivation probably similar to the above. In the Quendi and Eldar essay from 1959-60 it was named Brithonbar with an n (WJ/380), but this may simply have been a slip.
Brithon (name of the river “pebbly” [HKF]) + (m-)bar (“dwell, inhabit”)