An (archaic) word for “brother” in The Etymologies of the 1930s derived from the root ᴹ√TOR of the same meaning, with an irregular plural terein (Ety/TOR). In ordinary speech, it was replaced by muindor, with an initial element muin “dear”.
Neo-Sindarin: In notes from the late 1960s, Tolkien introduced a new word hanar for “brother” (VT47/14). However, I think †tôr and related words might be retained to mean a “metaphorical brother”, a close male associate who may or may not be related by blood, as with such words as gwador “(sworn) brother, associate”. In this paradigm, I would assume muindor still refers to a brother by blood, with an added connotation of strong affection.
An adjective in The Etymologies of the 1930s glossed “swooping, leaping down” derived from the root ᴹ√THOR “come swooping down” (Ety/THOR).