(More fully "March brown fly") any of various large brown mayflies, including (British) Rhithrogena haarupi and Ecdyurus venosus; an artificial fly imitating these.
Origin
Mid 19th century.
Definition of March brown in US English:
March brown
noun
(More fully "March brown fly") any of various large brown mayflies, including (British) Rhithrogena haarupi and Ecdyurus venosus; an artificial fly imitating these.