Definition of old–man's beard
1 : any of several clematises (especially Clematis vitalba) having plumose styles
2 : a greenish-gray pendulous lichen (Usnea barbata) growing on trees
First Known Use of old–man's beard
Medical Definition of old–man's beard
: a greenish gray pendulous lichen of the genus Usnea (U. barbata) that grows on trees and is a source of usnic acid
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