köken: From Middle Persian [script needed] (bwl /bōr/, “reddish-brown, bay [horse]”), from Proto-Iranian *buHráh, from Proto-Indo-Iranian *bʰruHrás, ultimately from Proto-Indo-European *bʰerH- (“brown”). Cognate with Ossetian бур (bur, “yellow”), Northern Kurdish bor (“grey”), Shughni vůr (“brown, grey”), and the Old Armenian loanword բորենի (boreni, “hyena”).
köken: Compound of 硼 (hō, “borax”) + 素 (so, “element”). First used by Ishiguro Tadanori in Kagaku Kunmō in 1870, probably from 蓬(ほう)砂(しゃ)素(そ) (hōshaso) in Seimi Kaisō by Udagawa Yōan, which is a calque of Dutch boraxstof.
köken: Inherited from Old Czech bor, from Proto-Slavic *borъ. Because in Slovak and Polish dialects it means "marsh", it is sometimes associated with Proto-Slavic *bara, meaning the same. More probably it is connected with Proto-Indo-European *bʰor-u-. Compare also borůvka.