What is the origin of bilious?

Bilious “extremely unpleasant or distasteful” comes from Latin bīliōsus, which is formed from the noun bīlis “bile” and the suffix -ōsus “full of, containing.” The origins of bīlis are mysterious, but that has not stopped linguists from developing theories. One hypothesis, however flimsy, is a connection to Latin fel “gallbladder; venom, poison,” which could point to a derivation from the Proto-Indo-European root bhel- “to shine; white, yellow,” but this is a bit of a stretch of phonetic logic. Another option, in light of some potential cognates of bīlis in Celtic languages (such as Welsh bustl “gall, bile”), could point to a pre-Indo-European source from which only the Celtic and Italic languages borrowed the term. Alternatively, bīlis and its Celtic cognates could be distantly related to English bite because of bile’s digestive effects. Bilious was first recorded in English circa 1540.