The Collaborative International Dictionary of English v.0.48:

Hay \Hay\, n. [OE. hei, AS. h[=e]g; akin to D. hooi, OHG. hewi,
   houwi, G. heu, Dan. & Sw. h["o], Icel. hey, ha, Goth. hawi
   grass, fr. the root of E. hew. See Hew to cut.]
   Grass cut and cured for fodder.
   [1913 Webster]

         Make hay while the sun shines.           --Camden.
   [1913 Webster]

         Hay may be dried too much as well as too little. --C.
                                                  L. Flint.
   [1913 Webster]

   Hay cap, a canvas covering for a haycock.

   Hay fever (Med.), nasal catarrh accompanied with fever, and
      sometimes with paroxysms of dyspn[oe]a, to which some
      persons are subject in the spring and summer seasons. It
      has been attributed to the effluvium from hay, and to the
      pollen of certain plants. It is also called hay asthma,
      hay cold, rose cold, and rose fever.

   Hay knife, a sharp instrument used in cutting hay out of a
      stack or mow.

   Hay press, a press for baling loose hay.

   Hay tea, the juice of hay extracted by boiling, used as
      food for cattle, etc.

   Hay tedder, a machine for spreading and turning new-mown
      hay. See Tedder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hay fever
    n 1: a seasonal rhinitis resulting from an allergic reaction to
         pollen [syn: hay fever, pollinosis]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "hay fever":
   Asiatic flu, Hong Kong flu, acute bronchitis, adenoiditis,
   allergen, allergic disorder, allergy, aluminosis, amygdalitis,
   anthracosilicosis, anthracosis, asbestosis, asthma,
   atypical pneumonia, bituminosis, black lung, bronchial pneumonia,
   bronchiectasis, bronchiolitis, bronchitis, bronchopneumonia,
   catarrh, chalicosis, chronic bronchitis, cold, collapsed lung,
   common cold, coniosis, conjunctivitis, coryza, cosmetic dermatitis,
   croup, croupous pneumonia, double pneumonia, dry pleurisy, eczema,
   emphysema, empyema, epidemic pleurodynia, fibrinous pneumonia, flu,
   grippe, hives, influenza, la grippe, laryngitis, lipoid pneumonia,
   lobar pneumonia, lung cancer, lung fever, pharyngitis, pleurisy,
   pleuritis, pneumococcal pneumonia, pneumoconiosis, pneumonia,
   pneumonic fever, pneumothorax, pollinosis, quinsy, rheum,
   rose cold, siderosis, silicosis, sore throat, swine flu,
   the sniffles, the snuffles, tonsilitis, urticaria, virus pneumonia,
   wet pleurisy, whooping cough