Source : Webster's Revised Unabridged Dictionary (1913)

Ail \Ail\ ([=a]l), v. t. [imp. & p. p. {Ailed} ([=a]ld); p. pr.
   & vb. n. {Ailing}.] [OE. eilen, ailen, AS. eglan to trouble,
   pain; akin to Goth. us-agljan to distress, agls troublesome,
   irksome, aglo, aglitha, pain, and prob. to E. awe. [root]3.]
   To affect with pain or uneasiness, either physical or mental;
   to trouble; to be the matter with; -- used to express some
   uneasiness or affection, whose cause is unknown; as, what
   ails the man? I know not what ails him.

         What aileth thee, Hagar?                 --Gen. xxi.
                                                  17.

   Note: It is never used to express a specific disease. We do
         not say, a fever ails him; but, something ails him.

Source : WordNet®

ailing
     adj : 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: {indisposed},
            {peaked(p)}, {poorly(p)}, {sickly}, {unwell}, {under
           the weather}]