Hero of Gondor and one of the Followship of the Ring (LotR/240). This name was also used by one of the Edain, 4th chief of the House of Bëor (S/148) and the 11th ruling steward of Gondor (LotR/1039). This name is a mixture of Sindarin and Quenya elements (LotR/1128), likely [N.] bôr “steadfast” and Q. mírë “jewel”. As pure Sindarin name, a final element S. mîr would have been lenited to -vir, producing ✱Borovir.
Conceptual Development: In Silmarillion drafts from the 1930s, N. Boromir was given as the name of an Easterling in Beleriand, one of the sons of Bór (LR/134). In The Etymologies, this name is explained as a combination of ON. boron “steadfast” and mīro (a masculinized form of ON. mīre “jewel”), thus ON. Boronmīro > N. Boromir (Ety/BOR, MIR). In this earlier etymology, the preceding n would prevented the mutation of the m. In Silmarillion revisions from the 1950s-60s, this character’s name changed to Borlach (WJ/240).
In The Lord of the Rings, S. Boromir was not longer an ancient name. This may be why Tolkien changed it to a mixed-language name to explain the presence of the m. However, the earlier etymology could have been applicable to (archaic) S. Boromir of the House of Bëor from The Silmarillion.
Earlier name of Borlach (LR/134). In The Etymologies, it was given as a derivative of ON. Boronmīro (Ety/BOR). The medial [m] developed from [mm] < [n+m], which is why it did not become [v]. This is a distinct etymology of the later name of S. Boromir of Gondor, where the medial [m] was explained by the fact that it was a mixture of Sindarin and Quenya elements.