boldyoudaoicibaDictYouDict[bold 词源字典]
bold: [OE] In Old English, bold meant simply ‘brave’; the modern connotations of immodesty or presumptuousness do not seem to have developed until the 12th century. The word goes back to a prehistoric Germanic *balthaz, based ultimately, it has been speculated, on Indo- European *bhel- ‘swell’ (the psychological link through ‘being puffed up’ via ‘adventurous courage’ to ‘audacity’ is scarcely far-fetched). If this is so it would mean bold is related to bellows, belly, billow, bolster, and possibly bellow and bell. The notion of impetuosity is perhaps retained in the related German bald ‘soon’.
=> bell, bellows, belly, billow, bolster[bold etymology, bold origin, 英语词源]
bold (adj.)youdaoicibaDictYouDict
Old English beald (West Saxon), bald (Anglian) "bold, brave, confident, strong," from Proto-Germanic *balthaz (cognates: Old High German bald "bold, swift," in names such as Archibald, Leopold, Theobald; Gothic balþei "boldness;" Old Norse ballr "frightful, dangerous"), perhaps from PIE *bhol-to- suffixed form of *bhel- (2) "to blow, swell" (see bole).

Of flavors (coffee, etc.) from 1829. The noun meaning "those who are bold" is from c. 1300. Old French and Provençal baut "bold," Italian baldo "bold, daring, fearless" are Germanic loan-words.