From Proto-Italic *bistli-. De Vaan notes that the word is only found in Latin and Brythonic (Welsh bustl) and does not rule out a substrate borrowing for both; however, it could instead be a derivative of a preform *bid-tli-, from Proto-Indo-European *bʰeyd- (“to split off, bite”), referring to the substance's bitterness. In the latter case, compare also Proto-Germanic *bitraz (“bitter”).