Mid 19th century; earliest use found in John Henry Newman (1801–1890), theologian and cardinal. From the name of Plymouth + brother, after Plymouth Brethren.
Definition of Plymouth brother in US English:
Plymouth brother
nounˌplɪməθ ˈbrʌðə
A member of the Plymouth Brethren.
Origin
Mid 19th century; earliest use found in John Henry Newman (1801–1890), theologian and cardinal. From the name of Plymouth + brother, after Plymouth Brethren.