طحمآء doesn't appear in Hans Wehr's Modern Standard Arabic dictionary, but Lane's Arabic-English Lexicon has an entry for it. Select the entry below for the full classical Arabic definition.

• طَحْمَآءُ

  طَحْمَآءُ ذ A species of plant, (S, K,) growing in plain, or soft, land, of the kind called حَمْض: (TA:) or i. q. نَجِيلٌ, (AHn, K, TA,) which is the best of all the حَمْض, [herbaceous, i. e.] not having wood fit for fu…

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