The Collaborative International Dictionary of English v.0.48:

Weather \Weath"er\, n. [OE. weder, AS. weder; akin to OS. wedar,
   OFries. weder, D. weder, we[^e]r, G. wetter, OHG. wetar,
   Icel. ve[eth]r, Dan. veir, Sw. v[aum]der wind, air, weather,
   and perhaps to OSlav. vedro fair weather; or perhaps to Lith.
   vetra storm, Russ. vieter', vietr', wind, and E. wind. Cf.
   Wither.]
   [1913 Webster]
   1. The state of the air or atmosphere with respect to heat or
      cold, wetness or dryness, calm or storm, clearness or
      cloudiness, or any other meteorological phenomena;
      meteorological condition of the atmosphere; as, warm
      weather; cold weather; wet weather; dry weather, etc.
      [1913 Webster]

            Not amiss to cool a man's stomach this hot weather.
                                                  --Shak.
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            Fair weather cometh out of the north. --Job xxxvii.
                                                  22.
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   2. Vicissitude of season; meteorological change; alternation
      of the state of the air. --Bacon.
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   3. Storm; tempest.
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            What gusts of weather from that gathering cloud
            My thoughts presage!                  --Dryden.
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   4. A light rain; a shower. [Obs.] --Wyclif.
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   Stress of weather, violent winds; force of tempests.

   To make fair weather, to flatter; to give flattering
      representations. [R.]

   To make good weather, or To make bad weather (Naut.), to
      endure a gale well or ill; -- said of a vessel. --Shak.

   Under the weather, ill; also, financially embarrassed.
      [Colloq. U. S.] --Bartlett.

   Weather box. Same as Weather house, below. --Thackeray.

   Weather breeder, a fine day which is supposed to presage
      foul weather.

   Weather bureau, a popular name for the signal service. See
      Signal service, under Signal, a. [U. S.]

   Weather cloth (Naut.), a long piece of canvas of tarpaulin
      used to preserve the hammocks from injury by the weather
      when stowed in the nettings.

   Weather door. (Mining) See Trapdoor, 2.

   Weather gall. Same as Water gall, 2. [Prov. Eng.]
      --Halliwell.

   Weather house, a mechanical contrivance in the form of a
      house, which indicates changes in atmospheric conditions
      by the appearance or retirement of toy images.
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            Peace to the artist whose ingenious thought
            Devised the weather house, that useful toy!
                                                  --Cowper.
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   Weather molding, or

   Weather moulding (Arch.), a canopy or cornice over a door
      or a window, to throw off the rain.

   Weather of a windmill sail, the obliquity of the sail, or
      the angle which it makes with its plane of revolution.

   Weather report, a daily report of meteorological
      observations, and of probable changes in the weather;
      esp., one published by government authority.

   Weather spy, a stargazer; one who foretells the weather.
      [R.] --Donne.

   Weather strip (Arch.), a strip of wood, rubber, or other
      material, applied to an outer door or window so as to
      cover the joint made by it with the sill, casings, or
      threshold, in order to exclude rain, snow, cold air, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

under the weather
    adj 1: somewhat ill or prone to illness; "my poor ailing
           grandmother"; "feeling a bit indisposed today"; "you look
           a little peaked"; "feeling poorly"; "a sickly child"; "is
           unwell and can't come to work" [syn: ailing,
           indisposed, peaked(p), poorly(p), sickly,
           unwell, under the weather, seedy]