Noun
Yellow Boy (plural Yellow Boys)
(firearms, informal) A Winchester Model 1866 lever action rifle.
(countable, slang, now archaic, historical) A gold coin; a guinea or sovereign. [from 17th c.]
(uncountable, slang) Iron(III) hydroxide, a yellow-orange solid found in acidic water from mining projects.
(countable, US, slang, historical) Synonym of goldback (“kind of gold certificate”)
Source: en.wiktionary.org