تناهضوا 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.

• تناهضوا Form 6

  6 تناهضوا فِى الحَرْبِ ذ (S, A, Mgh, K) They rose, one with another, or one against another, and withstood, or opposed, one another. in war. or battle: (Mgh,) or each party of them rose and hastened (نَهَضَ) to, or towar…

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