Brass Monkey Meaning

(idiomatic, of the weather) Very cold.

Example:   It's brass monkey weather today, isn't it?
1987, Brian Carter, Jack:
  This is brass monkey weather and it'll get worse.
1997, Maeve Haran, A family affair:
  I forgot it'd be brass monkey weather in good old London.
2005, John G. Watson, The Golden Ball, page 115:
  Had to milk cows besides, and them winters up there in Wisconsin is brass monkey cold.