صدأ 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.

• صَدِئَ Form 1

  1 صَدِئَ ذ , (S, M, L, K,) aor. صَدَاَ , (L,) inf. n. صَدَأٌ, (S, M,) said of a horse, (K, TA,) and of a kid, (S, TA,) [or a goat,] He was of the colour termed صُدْأَة [i. e. sorrel inclining to blackness; or blackness i…

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• صَدَأٌ

  صَدَأٌ ذ inf. n. of صَدِئَ [q. v.]. (S, M.) -b2- Also [a subst.] signifying The rust of iron, (S, M, * TA,) and of copper and the like. (Har p. 481. [But there erroneously written صدآء.]) -A2- Also A man slender in body…

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