رس doesn't appear in Hans Wehr's Modern Standard Arabic dictionary, but Lane's Arabic-English Lexicon has 2 entries for it, distinguished by diacritics. Select one below for the full classical Arabic definition.

• رَسَّ Form 1

  1 رَسَّ الهُوَى فِى قَلْبِهِ ذ , and السُّقْمُ فِى جِسْمِهِ, [aor., accord. to the general rule, رَسِسَ ,] inf. n. رَسٌّ and رَسيسٌ, Love entered, and established itself, in his heart, and disease in his body; as also . …

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• رَسٌّ

  رَسٌّ ذ The beginning, or commencement, of a thing. (K.) And hence, (K,) رَسُّ الحُمَّى, and , The beginning, or commencement, (M, A, K,) or first touch, (S,) of fever, (S, M, A, K,) before it becomes vehement, or severe…

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