The Collaborative International Dictionary of English v.0.48:

Unwell \Un*well"\, a.
   1. Not well; indisposed; not in good health; somewhat ill;
      ailing.
      [1913 Webster]

   2. (Med.) Specifically, ill from menstruation; affected with,
      or having, catamenial; menstruant.
      [1913 Webster]

   Note: This word was formerly regarded as an Americanism, but
         is now in common use among all who speak the English
         language.
         [1913 Webster]

WordNet (r) 3.0 (2006):

unwell
    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]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "unwell":
   ailing, bad, below par, critically ill, donsie, down, faint,
   faintish, feeble, feeling awful, feeling faint,
   feeling something terrible, frail, ill, in danger, indisposed,
   infirm, laid low, low, mean, mortally ill, not quite right,
   off-color, offish, out of sorts, poorly, qualmish, queasy, rocky,
   seedy, shaky, sick, sick unto death, sickish, sickly, squeamish,
   taken ill, under the weather, weakly, wobbly