(An affectionate name for) Queen Elizabeth, the Queen Mother (1900–2002), consort of King George VI of Great Britain and Northern Ireland and mother of Queen Elizabeth II.
Origin
1950s; earliest use found in New York Daily News.
Definition of Queen Mum in US English:
Queen Mum
noun
informal
(An affectionate name for) Queen Elizabeth, the Queen Mother (1900–2002), consort of King George VI of Great Britain and Northern Ireland and mother of Queen Elizabeth II.