صيف 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.

• صَافَ Form 1

  1 صَافَ ذ , (S, M, O, Msb, K,) aor. يَصِيفُ, inf. n. صَيْفٌ, (TA,) He, or it, (a company of men, M, Msb,) remained, stayed, dwelt, or abode, (S, M, O, Msb, K,) during the [season called] صَيْف, (S, O,) or during his, or …

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• صَيْفٌ

  صَيْفٌ ذ as signifying A certain portion of the year is said by ISk to be fem.: (TA, voce شِتَآءٌ; q. v.:) [but by others I find it treated as masc.:] the صَيْف as meaning one of the seasons is well known: (M:) Lth says…

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