Definition of yellow boy in English:

yellow boy

Pronunciation /ˌjɛləʊ ˈbɔɪ/

noun

slang, archaic, historical
  • A gold coin, especially a guinea or a sovereign.

Origin

Mid 17th century; earliest use found in Edmund Gayton (1608–1666), physician and hack writer. From yellow + boy, with allusion to the coin's colour.