A person who wears a blue shirt, especially (frequently with capital initials) as a sign of allegiance to or membership of a particular group, team, political party, etc.
Origin
Mid 19th century.
Definition of blue shirt in US English:
blue shirt
noun
A person who wears a blue shirt, especially (frequently with capital initials) as a sign of allegiance to or membership of a particular group, team, political party, etc.