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Meaning of lady's-thumb

la•dy's-thumb

Pronunciation: (lā'dēz-thum"), [key]
— n.
  1. a smartweed, Polygonum persicaria, of the buckwheat family, having pink or purplish flowers and lance-shaped leaves with a spot resembling a thumbprint.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.