"Old Lady" is a colloquial term for the Bank of England, the United Kingdom's central bank.
The nickname comes from a satirical cartoon published in 1797 by James Gillray.
The comic is titled "Political Plunder, or Threadneedle Street's Old Lady in Danger! " and depicts the Bank of England as an old lady being attacked by politicians.
The Bank of England is located in Threadneedle Street, London, which is where the nickname "The Old Lady of Threadneedle Street" comes from.
Over time the term has become a widely recognized and affectionate nickname for the Bank of England.