"of a dull red color, reddish-brown," 1781, from Latin rufus "red, reddish, tawny, red-haired," from an Osco-Umbrian cognate of Latin ruber "red" (from PIE root *reudh- "red, ruddy"). Mostly in names or descriptions of birds or other animals; sometimes frowned upon in early use as just a French word for "reddish." Related: Rufulous.
Proto-Indo-European root meaning "red, ruddy." The only color for which a definite common PIE root word has been found. The initial -e- in the Greek word is because Greek tends to avoid beginning words with -r-.
It is the hypothetical source of/evidence for its existence is provided by: Latin ruber, also dialectal rufus "light red," mostly of hair; Greek erythros; Sanskrit rudhira-; Avestan raoidita-; Old Church Slavonic rudru, Polish rumiany, Russian rumjanyj "flushed, red," of complexions, etc.; Lithuanian raudas; Old Irish ruad, Welsh rhudd, Breton ruz "red."