The history of the word ‘bad’, Chapter 2
Quite often the first solid etymology of an English word comes from Skeat, but this is not the case with the adjective bad. In the first edition of his dictionary (1882), he could offer, with much hesitation, two Celtic cognates of bad, one of them being Irish Gaelic baodh “vain, giddy, foolish, simple.” Much later, Charles Mackay, who believed that Irish Gaelic was the source of most English words, mentioned beud “mischief, hurt” as the etymon of bad.