يأس 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.

• يَئِسَ Form 1

  1 يَئِسَ ذ , (S, M, A, Msb, K,) مِنَ الشَّىْءِ, (S, A *, Msb,) aor. يَيْئَسُ and يَيْئِسُ, (S, M, Msb, K,) the latter of which is extr., (Sb, S, M, K,) like يَحْسِبُ, aor. of حَسِبَ, and يَنْعِمُ, aor. of نَعِمَ, (As, S,…

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• يَأَسٌ

  يَأَسٌ ذ Phthisis, or consumption; syn. سِلٌّ; (M, K;) because he who is affected by it is despaired of; (M;) or [because] the first who was affected by it was اليَأَسُ, (K, TA,) or إِلْيَاس, (TA,) the son of Mudar the s…

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