The Collaborative International Dictionary of English v.0.48:

Sickish \Sick"ish\, a.
   1. Somewhat sick or diseased.
      [1913 Webster]

   2. Somewhat sickening; as, a sickish taste.
      [1913 Webster] -- Sick"ish*ly, adv. -- Sick"ish*ness,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sickish
    adj 1: feeling nausea; feeling about to vomit [syn: nauseated,
           nauseous, queasy, sick, sickish]