Source : Webster's Revised Unabridged Dictionary (1913)

Sickly \Sick"ly\, a. [Compar. {Sicklier}; superl. {Sickliest}.]
   1. Somewhat sick; disposed to illness; attended with disease;
      as, a sickly body.

            This physic but prolongs thy sickly days. --Shak.

   2. Producing, or tending to, disease; as, a sickly autumn; a
      sickly climate. --Cowper.

   3. Appearing as if sick; weak; languid; pale.

            The moon grows sickly at the sight of day. --Dryden.

            Nor torrid summer's sickly smile.     --Keble.

   4. Tending to produce nausea; sickening; as, a sickly smell;
      sickly sentimentality.

   Syn: Diseased; ailing; infirm; weakly; unhealthy; healthless;
        weak; feeble; languid; faint.

Source : WordNet®

sickly
     adj 1: unhealthy looking [syn: {sallow}]
     2: 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)}, {unwell}, {under the weather}]
     [also: {sickliest}, {sicklier}]

sickliest
     See {sickly}