badius.

Superficially appears to be from late Proto-Indo-European *bodyos (“yellow, brown”) (though De Vaan rejects medial *-dy-, instead preferring *b⁽ʰ⁾odʰyos) with unrounding of *bo- to Latin *ba-, as it is clearly cognate to Old Irish buide (“yellow”) (from Proto-Celtic *bodyos). However, the word's limited distribution, difficult phonetics and absence of a known root source all render Proto-Indo-European origin unlikely. Rather there may be a borrowing relationship between Italic and Celtic or even a substrate source; ultimate origin unknown. The immediate source of badius may well be Gaulish *badios; compare Latin Baius. The semantic narrowing in Latin to a word for horses also favors the Celtic origin as a Kulturwort.