Definition of brass monkey in English:

brass monkey

noun

informal
  • Used in phrases to refer to extremely cold weather:

    ‘it's brass monkey weather tonight’

Origin

Mid 19th century: often said to be from a type of brass rack or monkey in which cannonballs were stored and which contracted in very cold weather, ejecting the balls, but this explanation has not been proved.

Pronunciation

brass monkey

/ˌbrɑːs ˈmʌŋki/