Definition of old man's beard
noun [mass noun]
1a wild clematis which has grey fluffy hairs around the seeds.
- Genus Clematis, family Ranunculaceae: several species, in particular (in Britain) traveller’s joy
2a large lichen that forms shaggy greyish beard-like growths on the branches of trees.
- Usnea barbata and related species, order Parmeliales