A polypore (bracket fungus), Piptoporus betulinus (family Fomitopsidaceae), that grows on decaying birch trees and was formerly used for stropping razors.
Origin
Late 19th century.
Definition of razor strop fungus in US English:
razor strop fungus
noun
A polypore (bracket fungus), Piptoporus betulinus (family Fomitopsidaceae), that grows on decaying birch trees and was formerly used for stropping razors.