"There may have been an Old English *bulla, whence the derivative bulluc ‘bullock’, as the source of the Middle English bule, bulle, and the modern bull, which do not fit phonetically the bole forms" (OED)
Buck, C., D. (1949). A dictionary of selected synonyms in the principal Indo–European languages: a contribution to the history of ideas. Chicago: University of Chicago Press.
Guus Kroonen. Etymological Dictionary of Proto-Germanic. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online.
De Vries, Jan (1977). Altnordisches etymologisches Wörterbuch. 3rd ed. Leiden: Brill
Simpson, J., & Weiner, E. S. 1989. Oxford English dictionary. Oxford: Clarendon Press.
Young, George Vaughan Chichester & Clewer, Cynthia R. (1985). Føroysk-Ensk ordabók: Faroese-English dictionary: with Faroese folk-lore and proverbs: and a section by WB Lockwood on Faroese pronunciation. Peel, Isle of Man: Mansk-Svenska Publ. Co.
Collins German dictionary.. 5th ed. (2004). London: Collins
Karulis, Konstantīns (2001). Latviešu etimoloǧijas vārdnīca. Rīgā: Avots
Robert Farren, PIE culture words collection, 2017
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